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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:
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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): 20.0.14
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.29
PHP version (eg, 7.4): 7.4.27
The issue you are facing:
I decided to update my nextcloud instance (running for long time - sinf version 12 i guess) from version 19.x to the most actual one.
First step, was to newest version 19 → success (via web-gui)
Second: Update from version 19 to 20.0.14 - success, but some warning regarding indexes and so on - I fixed them by execute the suggested occ commands. Also warning about php version 7.2 - that nextcloud 20 is the last version, that support this. Also one warning regarding one file, that has wrong hash (mimetypelist.js)
No more update were available from GUI - so I started to update php to version 7.4
After several steps, I saw that php 8.x is installed - everything worked, but still warning of “old version 7.2 in update screen”. I order to get also version 21, I moved on:
I did the mistake I think: I removed all php 7.x versions. After that nothing worked anymore.
Webseite shows: Service unavailable.
I reinstalled php version 7.4 . x
sudo apt-get install php7.4-mysql php7.4-curl php7.4-zip php7.4-mbstring php7.4-gd php7.4-intl
I also uninstalled php version 8.x
I am not a linux pro - I like nextcloud a lot, but this time I did something wrong, and I even dont know , into which logs I have to look into.
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:
Not accessible - because no GUI available
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'passwordsalt' => 'xxxx',
'secret' => 'xxxx',
'trusted_domains' =>
array (
0 => 'localhost',
1 => 'xxx-mylocalip-xxx',
2 => 'xxx-mydomain-xxx',
),
'datadirectory' => '/mnt/ncdata',
'overwrite.cli.url' => 'https://xxx-mydomain-xxx',
'dbtype' => 'pgsql',
'version' => '20.0.14.2',
'dbname' => 'nextcloud_db',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'dbuser' => 'ncadmin',
'dbpassword' => 'xxx',
'installed' => true,
'instanceid' => 'xxx',
'mail_smtpmode' => 'smtp',
'log_rotate_size' => '10485760',
'memcache.local' => '\\OC\\Memcache\\Redis',
'filelocking.enabled' => true,
'memcache.distributed' => '\\OC\\Memcache\\Redis',
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => '/var/run/redis/redis-server.sock',
'port' => 0,
'timeout' => 0.5,
'dbindex' => 0,
'password' => 'xxx',
),
'htaccess.RewriteBase' => '/',
'maintenance' => false,
'updater.release.channel' => 'beta',
'theme' => '',
'loglevel' => 2,
'cache_chunk_gc_ttl' => 10800,
'trashbin_retention_obligation' => 'auto',
'updater.secret' => 'xxx',
);
The output of your Apache/nginx/system log in /var/log/____ This errors were there before also!:
[Wed Feb 02 13:23:53.987442 2022] [proxy:error] [pid 2175] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php7.2-fpm.nextcloud.sock (*) failed
[Wed Feb 02 13:23:53.987480 2022] [proxy_fcgi:error] [pid 2175] [client xxx:52030] AH01079: failed to make connection to backend: httpd-UDS
[Wed Feb 02 13:24:23.955661 2022] [proxy:error] [pid 7639] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php7.2-fpm.nextcloud.sock (*) failed
[Wed Feb 02 13:24:23.956237 2022] [proxy_fcgi:error] [pid 7639] [client xxx:52035] AH01079: failed to make connection to backend: httpd-UDS
[Wed Feb 02 13:24:23.964450 2022] [proxy:error] [pid 2184] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /run/php/php7.2-fpm.nextcloud.sock (*) failed
[Wed Feb 02 13:24:23.964486 2022] [proxy_fcgi:error] [pid 2184] [client xxx:52036] AH01079: failed to make connection to backend: httpd-UDS