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Sorry to hear you’re facing problems 
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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, 18.0.2): 18.0.4
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): Nginx 1.17.10
PHP version (eg, 7.1): 7.4.6
The issue you are facing: Internal Server Error after updating to Ubuntu 20.04. I was previously experiencing the issue of login to the web interface fails on all attempts and redirects to login?redirect_url=/apps/files/ . At this stage I was still running Ubuntu 18.04 but have subsequently upgraded to 20.04 in an attempt to fix the issue.
Yesterday I installed PHP 7.4 and all required packages. I then stupidly removed PHP 7.3 and all it’s installed packages. At this point I was getting a 302 error in nginx. I them pointed my nginx config files to PHP 7.4 and got the web interface to show the login screen again. All of my desktop and phone clients worked at this point without issue but I still could not login to the web interface. All login attempts redirects to login?redirect_url=/apps/files/ . After being unable to resolve this issue today, I updated to Ubuntu 20.04 and now have an Internal Server error.
I had tried changing the permissions on /var/lib/php/sessions to both nginx:nginx and www-data:www-data (default is root:root) and this didn’t work either, despite other Topics with similar issues reporting that this resolved the issue.
What am I doing wrong? All help is greatly appreciated.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Installed PHP 7.4 and it’s associated packages (the same ones I had installed for 7.3)
- Uninstalled PHP 7.3 and it’s associated packages
- Can’t login to web interface and all login attempts redirect to login?redirect_url=/apps/files/ .
- Upgraded to Ubuntu 20.04
- Internal Server Error
The output of your Nextcloud log in Admin > Logging:
Please advise where I can find this in Ubuntu as I can't access the web interface.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'passwordsalt' => '',
'secret' => '',
'trusted_domains' =>
array (
0 => 'localhost',
1 => 'XX.XX.XXX.XX',
2 => 'nextcloud.test.com',
),
'datadirectory' => '/var/www/html/data',
'overwrite.cli.url' => 'http://localhost',
'dbtype' => 'mysql',
'version' => '18.0.4.2',
'dbname' => 'nextcloud',
'dbhost' => 'localhost',
0 => 'XX.XX.XXX.XX',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud12821',
'dbpassword' => '',
'installed' => true,
'memcache.local' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => 'localhost',
'port' => 6379,
),
'instanceid' => '',
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'enable_previews' => false,
'mail_from_address' => '',
'mail_smtpmode' => 'smtp',
'mail_smtpauthtype' => 'LOGIN',
'mail_domain' => '',
'updater.release.channel' => 'stable',
'app_install_overwrite' =>
array (
0 => 'contacts',
),
);
The output of your Apache/nginx/system log in /var/log/____:
Please advise exactly which log to provide and I'll provide it/them. I'm not sure if you require "access.log", "error.log", "nextcloud_https_access.log " or "nextcloud_https_error.log" from /var/log/nginx