Some admin task fail / can't add group to "Notify members..."

Hello,

Nextcloud 14.0.3.0
Fedora 28
Apache 2.4.34
PHP 7.2.11
This is behind another Apache 2.4 on Debian acting as reverse proxy.

one of my NextCloud instances shows - although working fine for the most part - one problem I am not able to solve:
Logged in as an administrator I go to Settings / Administration / Overview. If I select an additional group from the “Notify members…” pop-up the “Problem loading page…” info appears and after a couple of seconds the page reloads with the additional group gone from the “Notify
members…” field. Debug logging shows this:

{"reqId":"W9s0OKdmViblP2D6LG1r4AAAAMg","level":0,"time":"2018-11-01T17:13:28+00:00","remoteAddr":"<myip>","user":"xadmin","app":"core","method":"POST","url":"\/nextcloud\/ocs\/v2.php\/apps\/provisioning_api\/api\/v1\/config\/apps\/updatenotification\/notify_groups","message":{"Exception":"OC\\AppFramework\\Middleware\\Security\\Exceptions\\NotConfirmedException","Message":"Password confirmation is required","Code":403,"Trace":[{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Middleware\/MiddlewareDispatcher.php","line":95,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\Security\\PasswordConfirmationMiddleware","type":"->","args":[{"__class__":"OCA\\Provisioning_API\\Controller\\AppConfigController"},"setValue"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php","line":98,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\MiddlewareDispatcher","type":"->","args":[{"__class__":"OCA\\Provisioning_API\\Controller\\AppConfigController"},"setValue"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/App.php","line":118,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Provisioning_API\\Controller\\AppConfigController"},"setValue"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php","line":47,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OCA\\Provisioning_API\\Controller\\AppConfigController","setValue",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"app":"updatenotification","key":"notify_groups","_route":"ocs.provisioning_api.AppConfig.setValue"}]},{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler","type":"->","args":[{"app":"updatenotification","key":"notify_groups","_route":"ocs.provisioning_api.AppConfig.setValue"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/Route\/Router.php","line":297,"function":"call_user_func","args":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},{"app":"updatenotification","key":"notify_groups","_route":"ocs.provisioning_api.AppConfig.setValue"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/ocs\/v1.php","line":82,"function":"match","class":"OC\\Route\\Router","type":"->","args":["\/ocsapp\/apps\/provisioning_api\/api\/v1\/config\/apps\/updatenotification\/notify_groups"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/ocs\/v2.php","line":24,"args":["\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/ocs\/v1.php"],"function":"require_once"}],"File":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Middleware\/Security\/PasswordConfirmationMiddleware.php","Line":77,"CustomMessage":"--"},"userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}
{"reqId":"W9s0PlNthcQN1IDfHkYz-AAAAQ0","level":0,"time":"2018-11-01T17:13:34+00:00","remoteAddr":"<myip>","user":"xadmin","app":"no app in context","method":"GET","url":"\/nextcloud\/index.php\/settings\/admin\/overview","message":"No cache entry found for \/appdata_ocuff6ftudze\/avatar\/xadmin\/avatar.jpg (storage: local::\/srv\/apache2\/hosts/mycustomer\/example.com\/nc_mycustomer\/data\/, internalPath: appdata_ocuff6ftudze\/avatar\/xadmin\/avatar.jpg)","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}
{"reqId":"W9s0P1prdcFRITSthGtnDwAAAJg","level":0,"time":"2018-11-01T17:13:35+00:00","remoteAddr":"<myip>","user":"xadmin","app":"no app in context","method":"GET","url":"\/nextcloud\/index.php\/css\/icons\/icons-vars.css","message":"No cache entry found for \/appdata_ocuff6ftudze\/css\/icons\/icons-vars.css.gzip (storage: local::\/srv\/apache2\/hosts/mycustomer\/example.com\/nc_mycustomer\/data\/, internalPath: appdata_ocuff6ftudze\/css\/icons\/icons-vars.css.gzip)","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}
{"reqId":"W9s0P3itpLkkWCvVLQmgdAAAAEk","level":0,"time":"2018-11-01T17:13:35+00:00","remoteAddr":"<myip>","user":"xadmin","app":"no app in context","method":"GET","url":"\/nextcloud\/index.php\/avatar\/xadmin\/32?v=0","message":"No cache entry found for \/appdata_ocuff6ftudze\/avatar\/xadmin\/avatar.jpg (storage: local::\/srv\/apache2\/hosts/mycustomer\/example.com\/nc_mycustomer\/data\/, internalPath: appdata_ocuff6ftudze\/avatar\/xadmin\/avatar.jpg)","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}
{"reqId":"W9s0P3itpLkkWCvVLQmgdQAAAEI","level":0,"time":"2018-11-01T17:13:35+00:00","remoteAddr":"<myip>","user":"--","app":"webdav","method":"PROPFIND","url":"\/nextcloud\/remote.php\/dav","message":{"Exception":"Sabre\\DAV\\Exception\\NotAuthenticated","Message":"Cannot authenticate over ajax calls","Code":0,"Trace":[{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/lib\/Connector\/Sabre\/Auth.php","line":155,"function":"auth","class":"OCA\\DAV\\Connector\\Sabre\\Auth","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Auth\/Plugin.php","line":201,"function":"check","class":"OCA\\DAV\\Connector\\Sabre\\Auth","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Auth\/Plugin.php","line":150,"function":"check","class":"Sabre\\DAV\\Auth\\Plugin","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"function":"beforeMethod","class":"Sabre\\DAV\\Auth\\Plugin","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/event\/lib\/EventEmitterTrait.php","line":105,"function":"call_user_func_array","args":[[{"autoRequireLogin":true,"__class__":"Sabre\\DAV\\Auth\\Plugin"},"beforeMethod"],[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php","line":466,"function":"emit","class":"Sabre\\Event\\EventEmitter","type":"->","args":["beforeMethod",[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php","line":254,"function":"invokeMethod","class":"Sabre\\DAV\\Server","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/lib\/Server.php","line":293,"function":"exec","class":"Sabre\\DAV\\Server","type":"->","args":[]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/appinfo\/v2\/remote.php","line":35,"function":"exec","class":"OCA\\DAV\\Server","type":"->","args":[]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/remote.php","line":163,"args":["\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/appinfo\/v2\/remote.php"],"function":"require_once"}],"File":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/lib\/Connector\/Sabre\/Auth.php","Line":249,"CustomMessage":"--"},"userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}
{"reqId":"W9s0P6dmViblP2D6LG1r4wAAANA","level":0,"time":"2018-11-01T17:13:35+00:00","remoteAddr":"<myip>","user":"--","app":"webdav","method":"PROPFIND","url":"\/nextcloud\/remote.php\/dav","message":{"Exception":"Sabre\\DAV\\Exception\\NotAuthenticated","Message":"Cannot authenticate over ajax calls","Code":0,"Trace":[{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/lib\/Connector\/Sabre\/Auth.php","line":155,"function":"auth","class":"OCA\\DAV\\Connector\\Sabre\\Auth","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Auth\/Plugin.php","line":201,"function":"check","class":"OCA\\DAV\\Connector\\Sabre\\Auth","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Auth\/Plugin.php","line":150,"function":"check","class":"Sabre\\DAV\\Auth\\Plugin","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"function":"beforeMethod","class":"Sabre\\DAV\\Auth\\Plugin","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/event\/lib\/EventEmitterTrait.php","line":105,"function":"call_user_func_array","args":[[{"autoRequireLogin":true,"__class__":"Sabre\\DAV\\Auth\\Plugin"},"beforeMethod"],[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php","line":466,"function":"emit","class":"Sabre\\Event\\EventEmitter","type":"->","args":["beforeMethod",[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/3rdparty\/sabre\/dav\/lib\/DAV\/Server.php","line":254,"function":"invokeMethod","class":"Sabre\\DAV\\Server","type":"->","args":[{"absoluteUrl":"http:\/\/example.com\/nextcloud\/remote.php\/dav","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/lib\/Server.php","line":293,"function":"exec","class":"Sabre\\DAV\\Server","type":"->","args":[]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/appinfo\/v2\/remote.php","line":35,"function":"exec","class":"OCA\\DAV\\Server","type":"->","args":[]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/remote.php","line":163,"args":["\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/appinfo\/v2\/remote.php"],"function":"require_once"}],"File":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/apps\/dav\/lib\/Connector\/Sabre\/Auth.php","Line":249,"CustomMessage":"--"},"userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}

If I click “Log out” now, the log in page appears but only shows a “Access forbidden CSRF check failed” message and in the debug log there’s:

{"reqId":"W9s0QlprdcFRITSthGtnPQAAAJM","level":0,"time":"2018-11-01T17:13:39+00:00","remoteAddr":"<myip>","user":"xadmin","app":"core","method":"GET","url":"\/nextcloud\/index.php\/logout?requesttoken=52twc2A%2BgWM6Yq\/Ix%2BXAkrDQ%2B9zVkpaA519q4TULs\/g%3D%3AkSciQSR241sMCdWwhYCtpsaJlI7h5tH00wUDpnduhII%3D","message":{"Exception":"OC\\AppFramework\\Middleware\\Security\\Exceptions\\CrossSiteRequestForgeryException","Message":"CSRF check failed","Code":412,"Trace":[{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Middleware\/MiddlewareDispatcher.php","line":95,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\Security\\SecurityMiddleware","type":"->","args":[{"__class__":"OC\\Core\\Controller\\LoginController"},"logout"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php","line":98,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\MiddlewareDispatcher","type":"->","args":[{"__class__":"OC\\Core\\Controller\\LoginController"},"logout"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/App.php","line":118,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OC\\Core\\Controller\\LoginController"},"logout"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php","line":47,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OC\\Core\\Controller\\LoginController","logout",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"_route":"core.login.logout"}]},{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler","type":"->","args":[{"_route":"core.login.logout"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/Route\/Router.php","line":297,"function":"call_user_func","args":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},{"_route":"core.login.logout"}]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/base.php","line":987,"function":"match","class":"OC\\Route\\Router","type":"->","args":["\/logout"]},{"file":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/index.php","line":42,"function":"handleRequest","class":"OC","type":"::","args":[]}],"File":"\/srv\/apache2\/hosts/mycustomer\/example.com\/htdocs\/nextcloud\/lib\/private\/AppFramework\/Middleware\/Security\/SecurityMiddleware.php","Line":174,"CustomMessage":"--"},"userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}
{"reqId":"W9s0Q6dmViblP2D6LG1r5gAAAME","level":0,"time":"2018-11-01T17:13:39+00:00","remoteAddr":"<myip>","user":"xadmin","app":"no app in context","method":"GET","url":"\/nextcloud\/index.php\/css\/icons\/icons-vars.css","message":"No cache entry found for \/appdata_ocuff6ftudze\/css\/icons\/icons-vars.css.gzip (storage: local::\/srv\/apache2\/hosts/mycustomer\/example.com\/nc_mycustomer\/data\/, internalPath: appdata_ocuff6ftudze\/css\/icons\/icons-vars.css.gzip)","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:63.0) Gecko\/20100101 Firefox\/63.0","version":"14.0.3.0"}

If I navigate to some other page - for example “Files” - before clicking “Log out”, logging out works without error.

A second NextCloud instance on a different vhost on the same server behind the same reverse proxy doesn’t show this behaviour.

What might be the reason for this behaviour / what can I do to debug this?

Thanks in advance!

config.php:

<?php
$CONFIG = array (
  'passwordsalt' => 'salt',
  'secret' => 'secret',
  'trusted_domains' =>
  array (
    0 => 'localhost',
    1 => 'example.com',
  ),
  'datadirectory' => '/srv/apache2/hosts/mycustomer/example.com/nc_mycustomer/data',
  'memcache.local' => '\\OC\\Memcache\\Redis',
  'redis' =>
  array (
    'host' => 'localhost',
    'port' => 6379,
  ),
  'trashbin_retention_obligation' => 'auto, 30',
  'dbtype' => 'mysql',
  'version' => '14.0.3.0',
  'dbname' => 'nc_mycustomer',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'nc_mycustomer',
  'dbpassword' => 'password',
  'installed' => true,
  'instanceid' => 'instanceid',
  'mail_smtpmode' => 'smtp',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_from_address' => 'nc_mycustomer',
  'mail_domain' => 'example.com',
  'mail_smtpauth' => 1,
  'mail_smtphost' => 'smtp.ddt-consult.de',
  'mail_smtpport' => '587',
  'mail_smtpname' => 'oss_nc',
  'mail_smtppassword' => 'password',
  'mail_smtpsecure' => 'tls',
  'maintenance' => false,
#  'loglevel' => 0,
#  'logfile' => '/srv/apache2/hosts/mycustomer/example.com/nc_mycustomer/data/nc_debug.log',
  'theme' => '',
  'updater.release.channel' => 'stable',
  'overwrite.cli.url' => 'https://example.com/nextcloud',
);

No ideas, anyone?
Should I file a bug?