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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): 21.0.1.1
Operating system and version (eg, Ubuntu 20.04): Debian 10
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.38
PHP version (eg, 7.4): 7.3.27
The issue you are facing:
I have recently upgraded from Nextcloud 20 to 21 since then I can no longer access my files in the WebUI. When I access http://Nextcloud/apps/files the WebUI shows the following error:
Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
Everything else still works: file synchronization and mobile apps are still working and all other apps (dashboard, photos, settings, music, talk) are also working in the WebUI.
I don’t see anything in the Nextcloud log, but mysql is showing an aborted connection after accessing http://Nextcloud/apps/files
I have already tried rescanning all files (occ files:scan -all & files:cleanup) and reinstalling PHP with no success.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Update NC from 20 to 21
- Login to WebUI
- Go to Files
- See error: 503 Service Unavailable
The output of your Nextcloud log in Admin > Logging:
Empty
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => '*',
'passwordsalt' => '*',
'secret' => '*',
'trusted_domains' =>
array (
0 => '*',
),
'datadirectory' => '/media/data/nextcloud/data',
'overwrite.cli.url' => '*',
'dbtype' => 'mysql',
'version' => '21.0.1.1',
'dbname' => '*',
'dbhost' => '*',
'dbport' => '3306',
'dbtableprefix' => 'oc_',
'dbuser' => '*',
'dbpassword' => '*',
'logtimezone' => 'Europe/Berlin',
'log_type' => 'file',
'logfile' => '/var/log/nextcloud/nextcloud.log',
'loglevel' => 2,
'log_rotate_size' => 104857600,
'default_locale' => 'de_DE',
'default_phone_region' => 'DE',
'installed' => true,
'memcache.distributed' => '\\OC\\Memcache\\Redis',
'memcache.local' => '\\OC\\Memcache\\APCu',
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => '*',
'port' => 6381,
'password' => '*',
),
'mail_smtpmode' => 'smtp',
'mail_from_address' => '*',
'mail_domain' => '*',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpauth' => 1,
'mail_smtphost' => '*',
'mail_smtpport' => '587',
'mail_smtpname' => '*',
'mail_smtppassword' => '*',
'maintenance' => false,
'enable_previews' => true,
'enabledPreviewProviders' =>
array (
0 => 'OC\\Preview\\PNG',
1 => 'OC\\Preview\\JPEG',
2 => 'OC\\Preview\\GIF',
3 => 'OC\\Preview\\BMP',
4 => 'OC\\Preview\\XBitmap',
5 => 'OC\\Preview\\MP3',
6 => 'OC\\Preview\\TXT',
7 => 'OC\\Preview\\MarkDown',
8 => 'OC\\Preview\\Movies',
9 => 'OC\\Preview\\MSOffice2003',
10 => 'OC\\Preview\\MSOffice2007',
11 => 'OC\\Preview\\MSOfficeDoc',
12 => 'OC\\Preview\\OpenDocument',
13 => 'OC\\Preview\\PDF',
14 => 'OC\\Preview\\SVG',
15 => 'OC\\Preview\\TIFF',
16 => 'OC\\Preview\\Font',
17 => 'OC\\Preview\\Photoshop',
18 => 'OC\\Preview\\HEIC',
19 => 'OC\\Preview\\Image',
20 => 'OC\\Preview\\Movie',
21 => 'OC\\Preview\\MP4',
22 => 'OC\\Preview\\MOV',
23 => 'OC\\Preview\\MKV',
24 => 'OC\\Preview\\AVI',
25 => 'OC\\Preview\\JPG',
),
'theme' => '',
'updater.release.channel' => 'stable',
'mysql.utf8mb4' => true,
);
The output of your Apache log in /var/log/apache2/error.log:
[Tue Apr 20 17:33:14.111940 2021] [proxy_fcgi:error] [pid 32765:tid 2880580416] [client XXX.XXX.XXX.XXX:54344] AH01067: Failed to read FastCGI header
[Tue Apr 20 17:33:14.137941 2021] [proxy_fcgi:error] [pid 32765:tid 2880580416] (104)Connection reset by peer: [client XXX.XXX.XXX.XXX:54344] AH01075: Error dispatching request to :
[Tue Apr 20 17:33:18.403969 2021] [proxy_fcgi:error] [pid 32765:tid 2880580416] [client XXX.XXX.XXX.XXX:54346] AH01067: Failed to read FastCGI header
[Tue Apr 20 17:33:18.404033 2021] [proxy_fcgi:error] [pid 32765:tid 2880580416] (104)Connection reset by peer: [client XXX.XXX.XXX.XXX:54346] AH01075: Error dispatching request to :
The output of your MySql log in /var/log/mysql/error.log:
2021-04-20 17:33:14 2077 [Warning] Aborted connection 2077 to db: 'XXX' user: 'XXX' host: 'XXX' (Got an error reading communication packets)
2021-04-20 17:33:18 2079 [Warning] Aborted connection 2079 to db: 'XXX' user: 'XXX' host: 'XXX' (Got an error reading communication packets)