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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, 17.0.10):
Operating system and version (eg, Debian 10.6):
Apache or nginx version (eg, nginx 1.14.2):
PHP version (eg, 7.3):
The issue you are facing:
As admin, if I click “+ APPS” it keeps circling until time-out and then displays 504 Gateway time-out.
If I select administration->overview the updater keeps circling while checking for compatible apps.
Is this the first time you’ve seen this error? (Y):
Steps to replicate it:
I updated from 16.0.10. It required me to reload the page, since it could not find an update for the accessibility app. After reloading the update concluded and everything else seems to work properly.
The output of your Nextcloud log in Admin > Logging:
No log entries generated by either activity.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'xxxxx',
'passwordsalt' => 'xxxx',
'secret' => 'xxxx',
'trusted_domains' =>
array (
0 => 'xxx.mgr',
1 => 'xxx.de',
2 => 'xxx.de',
),
'datadirectory' => '/var/www/nextcloud/data',
'overwrite.cli.url' => 'https://xxx.de',
'dbtype' => 'mysql',
'version' => '17.0.10.1',
'dbname' => 'nextcloud',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => 'user',
'dbpassword' => 'secret',
'logtimezone' => 'UTC',
'installed' => true,
'mail_from_address' => 'cloud',
'mail_smtpmode' => 'smtp',
'mail_domain' => 'xxx.de',
'proxy' => 'xxx.de:8080',
//'user_backends' =>
//array (
// 0 =>
// array (
// 'class' => 'OC_User_SMB',
// 'arguments' =>
// array (
// 0 => 'xxx.de',
// ),
// ),
//),
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'mail_smtphost' => 'xxx.de',
'mail_sendmailmode' => 'smtp',
'updater.release.channel' => 'stable',
'mysql.utf8mb4' => true,
'updater.secret' => 'xxx',
);
The output of your Apache/nginx/system log in /var/log/____:
In /var/log/nginx/error.log I see various entries like the following for the updater spinning:
2020/10/08 22:50:51 [error] 27859#27859: *150499 access forbidden by rule, client: 172.16.1.2, server: cloud.XXX, request: "GET /data/.ocdata?t=1602190251377 HTTP/2.0", host: "cloud.xxx.de"
2020/10/08 22:57:13 [error] 27859#27859: *150499 access forbidden by rule, client: 172.16.1.2, server: cloud.xxx, request: "GET /data/.ocdata?t=1602190633312 HTTP/2.0", host: "cloud.xxx.de"
2020/10/08 22:59:14 [error] 27859#27859: *150499 access forbidden by rule, client: 172.16.1.2, server: cloud.mgr, request: "GET /data/.ocdata?t=1602190754087 HTTP/2.0", host: "cloud.xxx.de"
2020/10/08 23:00:14 [error] 27859#27859: *150499 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 172.16.1.2, server: cloud.xxx, request: "GET /ocs/v2.php/apps/updatenotification/api/v1/applist/18.0.10.2 HTTP/2.0", upstream: "fastcgi://unix:/run/php/cloud.sock", host: "cloud.xxx.de"
Nothing to see, when the APPS menu is selected.