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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, 12.0.2): 14.0.3
Operating system and version (eg, Ubuntu 17.04): Kubuntu 18.04.1
Apache or nginx version (eg, Apache 2.4.25): 2.4.29
PHP version (eg, 7.1): 7.2.10
The issue you are facing:
I’ve tried to configure the memroy cache according to the following instructions. This looked easy:
That didn’t work. Now, I can’t login to Nextcloud any longer. I get to the login screen, and can enter my name/password, but when I click on “Anmelden” (login), nothing at all happens.
I’ve restored the /var/www/html/nextcloud directory from a backup, but it stayed the same.
I couldn’t find any corresponding messages in /var/log/apache2/.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
Add caching to a working nextcloud installation according to the link above.
The output of your Nextcloud log in Admin > Logging:
I can’t get there any longer! See above.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'XXX',
'passwordsalt' => 'XXX',
'secret' => 'XXX',
'trusted_domains' =>
array (
0 => 'localhost',
1 => 'vwysk.now-dns.net',
),
'datadirectory' => '/var/www/html/nextcloud/data',
'dbtype' => 'sqlite3',
'version' => '14.0.3.0',
'overwrite.cli.url' => 'https://localhost/nextcloud',
'installed' => true,
'auth.bruteforce.protection.enabled' => false,
'maintenance' => false,
'mail_from_address' => 'post',
'mail_smtpmode' => 'smtp',
'mail_domain' => 'XXX',
'mail_smtpauth' => 1,
'mail_smtphost' => 'XXX',
'mail_smtpname' => 'XXX',
'mail_smtppassword' => 'XXX',
'mail_smtpport' => '25',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpsecure' => 'ssl',
'memcache.local' => '\\OC\\Memcache\\Redis',
'memcache.locking' => '\\OC\\Memcache\\Redis',
'filelocking.enabled' => 'true',
'redis' =>
array (
'host' => '/var/run/redis/redis-server.sock',
'port' => 0,
'timeout' => 0.0,
),
);
The output of your Apache/nginx/system log in /var/log/____:
Trying to login doesn’t produce any messages in /var/log/apache2/error.log.