Can't login on NextCloud 14 Web GUI

Nextcloud version (eg, 12.0.2): 14.0
Operating system and version (eg, Ubuntu 17.04): Ubuntu 18.04.1
Apache or nginx version (eg, Apache 2.4.25): 2.4.29
PHP version (eg, 7.1): 7.2.7

The issue you are facing:
Nobody can login via the web GUI. Desktop-Clients work fine so far. After entering the correct credentials, the button text changes and the circle appears for a second, then it reverts back to the login page with no error message. When entering wrong credentials, an error message is shown.

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Update NextCloud to 14.0

The output of your Nextcloud log in Admin > Logging:

{"reqId":"Qyf4gAsWToy9l3ro1IjM","level":0,"time":"2018-09-11T06:47:19+00:00","remoteAddr":"**redacted**","user":"--","app":"core","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":{"Exception":"OC\\AppFramework\\Middleware\\Security\\Exceptions\\NotLoggedInException","Message":"Current user is not logged in","Code":401,"Trace":[{"file":"\/var\/www\/html\/owncloud\/lib\/private\/AppFramework\/Middleware\/MiddlewareDispatcher.php","line":95,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\Security\\SecurityMiddleware","type":"->","args":[{"__class__":"OCA\\Files\\Controller\\ViewController"},"index"]},{"file":"\/var\/www\/html\/owncloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php","line":98,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\MiddlewareDispatcher","type":"->","args":[{"__class__":"OCA\\Files\\Controller\\ViewController"},"index"]},{"file":"\/var\/www\/html\/owncloud\/lib\/private\/AppFramework\/App.php","line":118,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Files\\Controller\\ViewController"},"index"]},{"file":"\/var\/www\/html\/owncloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php","line":47,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["ViewController","index",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"_route":"files.view.index"}]},{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler","type":"->","args":[{"_route":"files.view.index"}]},{"file":"\/var\/www\/html\/owncloud\/lib\/private\/Route\/Router.php","line":297,"function":"call_user_func","args":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},{"_route":"files.view.index"}]},{"file":"\/var\/www\/html\/owncloud\/lib\/base.php","line":989,"function":"match","class":"OC\\Route\\Router","type":"->","args":["\/apps\/files\/"]},{"file":"\/var\/www\/html\/owncloud\/index.php","line":42,"function":"handleRequest","class":"OC","type":"::","args":[]}],"File":"\/var\/www\/html\/owncloud\/lib\/private\/AppFramework\/Middleware\/Security\/SecurityMiddleware.php","Line":143,"CustomMessage":"--"},"userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit\/537.36 (KHTML, like Gecko) Chrome\/69.0.3497.81 Safari\/537.36","version":"14.0.0.19"}
{"reqId":"qWjSY95Jf3aii1hzPOJx","level":0,"time":"2018-09-11T06:47:19+00:00","remoteAddr":"**redacted**","user":"--","app":"no app in context","method":"GET","url":"\/owncloud\/index.php\/login?redirect_url=\/owncloud\/index.php\/apps\/files\/","message":"No cache entry found for \/appdata_oc1b9a530b21\/theming\/images\/background (storage: local::\/mnt\/owncloud_data\/, internalPath: appdata_oc1b9a530b21\/theming\/images\/background)","userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit\/537.36 (KHTML, like Gecko) Chrome\/69.0.3497.81 Safari\/537.36","version":"14.0.0.19"}

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'instanceid' => 'oc1b9a530b21',
  'passwordsalt' => '**redacted**',
  'trusted_domains' => 
  array (
    0 => '10.0.6.226',
    1 => '**redacted**',
    2 => '**redacted**',
  ),
  'datadirectory' => '/mnt/owncloud_data',
  'dbtype' => 'mysql',
  'version' => '14.0.0.19',
  'dbname' => 'ownclound',
  'dbhost' => 'localhost',
  'dbtableprefix' => 'oc_',
  'dbuser' => '**redacted**',
  'dbpassword' => '**redacted**',
  'installed' => true,
  'theme' => '',
  'default_language' => 'de',
  'maintenance' => false,
  'overwritewebroot' => '/owncloud',
  'overwrite.cli.url' => '/owncloud',
  'trashbin_retention_obligation' => '30, auto',
  'secret' => '**redacted**',
  'mail_smtpmode' => 'smtp',
  'mail_from_address' => 'iuk',
  'mail_domain' => 'le-mail.de',
  'mail_smtphost' => 'smtp.le-mail.de',
  'loglevel' => 0,
  'redis' => 
  array (
    'host' => 'localhost',
    'port' => 6379,
    'timeout' => 0.0,
    'password' => '',
  ),
  'memcache.local' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'asset-pipeline.enabled' => true,
  'singleuser' => false,
  'trusted_proxies' => 
  array (
    0 => '**redacted**',
  ),
  'forwarded_for_headers' => 
  array (
    0 => 'HTTP_X_FORWARDED_FOR',
  ),
  'data-fingerprint' => '61c92c7e5d6754191b33145032c3a5e5',
  'updater.secret' => '**redacted**',
);

The output of your Apache/nginx/system log in /var/log/____:
Nothing in error log

I reverted back to 13.0.5, you can delete this topic

Please join us at https://github.com/nextcloud/server/issues/10885 so that we can sort out this issue. Thank you.