Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
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): migration from 20 to 21
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04
Apache or nginx version (eg, Apache 2.4.25): nextcloud docker image
PHP version (eg, 7.4): nextcloud docker image
The issue you are facing:
After upgrading from 20 to 21 I cannot access the web interface (beyond login), the error in the logs is
172.19.0.25 - - [12/Apr/2021:20:38:02 +0000] "GET /apps/files/ HTTP/1.1" 500 4317 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.114 Safari/537.36"
Notes:
- I found earlier issues similar to this one suggesting to upgrade the metadata app. I do not use it and I manually updates all apps.
- the synchronization between the server and the Windows client works
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Upgrade from 20 to 21
- Try to login
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'htaccess.RewriteBase' => '/',
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'instanceid' => 'ocflnf1ody3y',
'passwordsalt' => 'REDACTED',
'secret' => 'REDACTED',
'trusted_domains' =>
array (
0 => 'nextcloud.REDACTED.eu',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'pgsql',
'version' => '21.0.1.1',
'overwrite.cli.url' => 'https://nextcloud.REDACTED.eu',
'overwriteprotocol' => 'https',
'auth.bruteforce.protection.enabled' => false,
'installed' => true,
'has_rebuilt_cache' => true,
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'mail_from_address' => 'nextcloud',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_domain' => 'swtk.info',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpauth' => 1,
'mail_smtphost' => 'smtp.REDACTED.fr',
'mail_smtpsecure' => 'ssl',
'mail_smtpport' => '465',
'mail_smtpname' => 'REDACTED',
'mail_smtppassword' => 'REDACTED',
'has_internet_connection' => true,
'twofactor_enforced' => 'true',
'twofactor_enforced_groups' =>
array (
0 => 'MFA',
),
'twofactor_enforced_excluded_groups' =>
array (
),
'filelocking.enabled' => true,
'trusted_proxies' =>
array (
0 => '172.19.0.31',
),
'forwarded_for_headers' =>
array (
0 => 'HTTP_X_FORWARDED_FOR',
),
'app_install_overwrite' =>
array (
0 => 'mindmap_app',
1 => 'twofactor_yubikey',
),
'dbname' => 'nextcloud',
'dbhost' => 'postgres',
'dbuser' => 'nextcloud',
'dbpassword' => 'REDACTED',
);