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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 21.0.4
Operating system and version (eg, Ubuntu 20.04): Linux 4.19.0-17-amd64 x86_64
Apache or nginx version (eg, Apache 2.4.25): 2.4.38
PHP version (eg, 7.4): 7.4.22
The issue you are facing:
Is this the first time you’ve seen this error? (Y/N):Y
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
{"reqId":"YSKN011xJ2YFJX18p9JGYQAAAAg","level":3,"time":"2021-08-22T17:48:03+00:00","remoteAddr":"+++++","user":"++++","app":"PHP","method":"GET","url":"/settings/admin/logging","message":{"Exception":"Error","Message":"file_exists(): open_basedir restriction in effect. File(/templates/) is not within the allowed path(s): (/var/customers/webs/****/nxtcloud:/tmp) at /var/customers/webs/****/nxtcloud/lib/private/Template/Base.php#68","Code":0,"Trace":[{"function":"onError","class":"OC\\Log\\ErrorHandler","type":"::"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/Template/Base.php","line":68,"function":"file_exists"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/legacy/OC_Template.php","line":147,"function":"getAppTemplateDirs","class":"OC\\Template\\Base","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/legacy/OC_Template.php","line":89,"function":"findTemplate","class":"OC_Template","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/TemplateLayout.php","line":92,"function":"__construct","class":"OC_Template","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/legacy/OC_Template.php","line":183,"function":"__construct","class":"OC\\TemplateLayout","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/public/AppFramework/Http/TemplateResponse.php","line":210,"function":"fetchPage","class":"OC_Template","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/AppFramework/Http/Dispatcher.php","line":172,"function":"render","class":"OCP\\AppFramework\\Http\\TemplateResponse","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/AppFramework/App.php","line":157,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/lib/private/Route/Router.php","line":302,"function":"main","class":"OC\\AppFramework\\App","type":"::"},{"file":"/var/customers/webs/****/nxtcloud/lib/base.php","line":993,"function":"match","class":"OC\\Route\\Router","type":"->"},{"file":"/var/customers/webs/****/nxtcloud/index.php","line":37,"function":"handleRequest","class":"OC","type":"::"}],"File":"/var/customers/webs/****/nxtcloud/lib/private/Log/ErrorHandler.php","Line":92,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.159 Safari/537.36","version":"21.0.4.1","id":"61228dd51497e"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => '1234',
'passwordsalt' => '1234+1234',
'secret' => '1234/1234+1234',
'trusted_domains' =>
array (
0 => 'cloud.1234.de',
),
'datadirectory' => '/var/customers/webs/1234/nxtcloud/data',
'dbtype' => 'mysql',
'version' => '21.0.4.1',
'overwrite.cli.url' => 'https://cloud.1234.de',
'dbname' => '1234',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => '1234',
'dbpassword' => '1234',
'installed' => true,
'mail_smtpmode' => 'smtp',
'mail_from_address' => 'cloudservice',
'mail_domain' => '1234e.com',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtphost' => 'localhost',
'mail_smtpport' => '25',
'maintenance' => false,
'default_phone_region' => 'DE',
'memcache.local' => '\\OC\\Memcache\\APCu',
'theme' => '',
'log_type' => 'file',
'logfile' => '/var/customers/webs/1234/nxtcloud/data/nextcloud2.log',
'loglevel' => 2,
'htaccess.RewriteBase' => '/',
'mail_sendmailmode' => 'smtp',
'mysql.utf8mb4' => true,
);
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE