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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, 12.0.2):18RC1
Operating system and version (eg, Ubuntu 17.04):CentOS 7.8
Apache or nginx version (eg, Apache 2.4.25): nginx 1.17.7
PHP version (eg, 7.1):7.4.1
The issue you are facing:
I see errors in the log, but everything feels ok.
Is this the first time you’ve seen this error? (Y/N):y
Steps to replicate it:
- Login
- At least when clicking images
- The errors appears
The output of your Nextcloud log in Admin > Logging:
{"reqId":"v8ovkg3mSiJ8tYdslmtk","level":3,"time":"2020-01-06T19:48:15+00:00","remoteAddr":"x","user":"admin","app":"PHP","method":"GET","url":"/apps/photos/","message":"Trying to access array offset on value of type null at /home/nginx/domains/public/3rdparty/leafo/scssphp/src/Compiler.php#5230","userAgent":"Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36","version":"18.0.0.8","id":"5e1391948523d"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => '13',
'passwordsalt' => 'Mountain salt',
'secret' => 'service',
'trusted_domains' =>
array (
0 => 'domains',
),
'datadirectory' => '/home/nginx/domains/data',
'dbtype' => 'mysql',
'version' => '18.0.0.8',
'overwrite.cli.url' => 'https://domains',
'dbname' => 'next',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'cloud',
'dbpassword' => 'qwerty',
'installed' => true,
'updater.release.channel' => 'beta',
'memcache.local' => '\OC\Memcache\Redis',
'filelocking.enabled' => 'true',
'memcache.locking' => '\OC\Memcache\Redis',
'redis' => array(
'host' => '/var/run/redis/redis.sock',
'port' => 0,
'timeout' => 0.0,
),
'log_rotate_size' => 1048576,
);
The output of your Apache/nginx/system log in /var/log/____:
21.12.33.99 - user [06/Jan/2020:21:48:05 +0200] "PROPFIND /remote.php/webdav/ HTTP/1.1" 401 355 "-" "Mozilla/5.0 (Android) Nextcloud-android/3.9.2"
21.12.33.99 - - [06/Jan/2020:21:48:05 +0200] "GET /apps/logreader/poll?lastReqId=WnzznQNw3LUIoR38j46i HTTP/2.0" 200 289 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:09 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 679 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:15 +0200] "GET /apps/logreader/poll?lastReqId=QyoMlZEXPwMcPzXQ0rj8 HTTP/2.0" 200 442 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:15 +0200] "GET /apps/photos/ HTTP/2.0" 200 5539 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:15 +0200] "GET /apps/logreader/poll?lastReqId=v8ovkg3mSiJ8tYdslmtk HTTP/2.0" 499 0 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /ocs/v2.php/apps/notifications/api/v2/notifications HTTP/2.0" 200 679 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /cron.php HTTP/2.0" 200 40 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "SEARCH /remote.php/dav/ HTTP/2.0" 207 2693 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /core/preview?fileId=10&x=256&y=256&a=false&v=e6964d1db7de4e7870c246aaeab3cfaa HTTP/2.0" 304 0 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /core/preview?fileId=11&x=256&y=256&a=false&v=346e4f6520c6706a900674bc15c35e54 HTTP/2.0" 304 0 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /core/preview?fileId=12&x=256&y=256&a=false&v=65134b3fda249dc55b4a69005aab5261 HTTP/2.0" 404 2 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /core/preview?fileId=8&x=256&y=256&a=false&v=969aead87c41dc793d25ce517e8eb29c HTTP/2.0" 304 0 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
21.12.33.99 - - [06/Jan/2020:21:48:16 +0200] "GET /core/preview?fileId=9&x=256&y=256&a=false&v=a8f1d42407a0c35c81af4e9780e948da HTTP/2.0" 304 0 "-" "Mozilla/5.0 (X11; CrOS x86_64 12607.58.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.86 Safari/537.36"
There are also `{"reqId":"1ocr1zneNOLFbHSfnhwX","level":3,"time":"2020-01-07T22:33:26+00:00","remoteAddr":"x","user":"admin","app":"PHP","method":"GET","url":"/apps/text/session/create?fileId=425&guestName=null&forceRecreate=false","message":"Trying to access array offset on value of type int at /home/nginx/domains/public/lib/private/Files/Node/Node.php#327","userAgent":"Mozilla/5.0 (Linux; Android 9; SM-G950F) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.93 Mobile Safari/537.36","version":"18.0.0.8","id":"5e1508f66be74"}```