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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): 19.0.6
Operating system and version (eg, Ubuntu 20.04): Ubuntu Server 20.04 LTS
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.41
PHP version (eg, 7.1): PHP 7.4
The issue you are facing:
We have upgraded our NextCloud instances from NextCloud 15 through to NextCloud 19. On 2x of our instances, we have a custom background image, which has been retained.
However, in one of our instances, we used the default NextCloud background, but changed the colour to a darker blue. Since upgrading NextCloud, the default background appears to have been replaced.
It previously had dots/circles in the background, which made it look less “plain” (example of the background but different colour is on the Social Login app in the App Store https://apps.nextcloud.com/apps/sociallogin), but has been replaced by a plain background in the same darker blue.
I have tried to look at replacing background.png in ./core/img and I looked for the file background in the data directory (./appdata_ocihlyxg8dpb\theming\images), but this didn’t exist (probably because we used the default background on this instance).
Please can someone advise how I can potentially get the previous background back? I do have backups of the /var/www/html/nextcloud directory before I performed the upgrades.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Upgrade NextCloud from NextCloud 15, through each major release to NextCloud 19
- Go to the login screen when maintenance mode is off
- Plain background instead of the previous “dotted” background
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
Whilst I'd happily paste this, the log is ridiculously long. If needed, please let me know, and I will try to paste a small snapshot of it.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'ocihlyxg8dpb',
'passwordsalt' => 'REMOVED VALUE'
'secret' => 'REMOVED VALUE'
'trusted_domains' =>
array (
0 => 'servername.domainname',
1 => 'url.domainname',
2 => 'servername',
3 => 'IP address',
),
'datadirectory' => '/remote-server/share',
'dbtype' => 'mysql',
'version' => '19.0.6.2',
'dbname' => 'dbname',
'dbhost' => 'localhost',
'dbtableprefix' => 'oc_',
'dbuser' => 'dbuser',
'dbpassword' => 'dbpassword',
'mysql.utf8mb4' => true,
'installed' => true,
'ldapIgnoreNamingRules' => false,
'mail_smtpmode' => 'smtp',
'mail_from_address' => 'REMOVED VALUE',
'mail_domain' => 'domain name',
'mail_smtphost' => 'smtp host',
'mail_smtpport' => '25',
'forcessl' => true,
'overwrite.cli.url' => 'https://url.domainname/instancename',
'overwriteprotocol' => 'https',
'logdateformat' => 'D, d M Y H:i:s',
'loglevel' => 2,
'logtimezone' => 'Europe/London',
'memcache.local' => '\\OC\\Memcache\\APCu',
'memcache.locking' => '\\OC\\Memcache\\Redis',
'redis' =>
array (
'host' => '/var/run/redis/redis-server.sock',
'port' => 0,
'timeout' => 0.0,
),
'default_language' => 'en',
'default_locale' => 'en_GB',
'theme' => '',
'skeletondirectory' => '',
'sharing.maxAutocompleteResults' => 0,
'sharing.minSearchStringLength' => 3,
'simpleSignUpLink.shown' => false,
'auth.webauthn.enabled' => false,
'maintenance' => false,
'updater.release.channel' => 'stable',
'updater.secret' => 'REMOVED VALUE',
);
The output of your Apache/nginx/system log in /var/log/____:
Whilst I'd happily paste the contents of error.log, from what I can see, entries for today are only from our security scanning system. Actual requests sent to load pages are not being logged in error.log. However, if needed, please let me know, and I will try to paste a snapshot of it.
A snapshot of contents of access.log (instance renamed to nextcloud)
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/css/guest.css?v=e65f1922-13 HTTP/1.1" 200 6182 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/index.php/apps/theming/styles?v=13 HTTP/1.1" 200 1474 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/index.php/js/core/merged-template-prepend.js?v=e65f1922-13 HTTP/1.1" 200 3958 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/search/js/search.js?v=e65f1922-13 HTTP/1.1" 200 2308 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/js/dist/main.js?v=e65f1922-13 HTTP/1.1" 200 425774 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/l10n/en_GB.js?v=e65f1922-13 HTTP/1.1" 200 6569 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/apps/files_sharing/l10n/en_GB.js?v=e65f1922-13 HTTP/1.1" 200 2808 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/apps/files_sharing/js/dist/main.js?v=e65f1922-13 HTTP/1.1" 200 1258 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/apps/files_pdfviewer/js/previewplugin.js?v=e65f1922-13 HTTP/1.1" 200 2878 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/index.php/apps/theming/image/logo?v=13 HTTP/1.1" 200 18728 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/search/js/searchprovider.js?v=e65f1922-13 HTTP/1.1" 200 4373 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/index.php/apps/theming/image/logo?useSvg=1&v=13 HTTP/1.1" 200 18728 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/js/files/fileinfo.js?v=e65f1922-13 HTTP/1.1" 200 1573 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/js/files/client.js?v=e65f1922-13 HTTP/1.1" 200 6425 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/core/js/dist/login.js?v=e65f1922-13 HTTP/1.1" 200 238276 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/index.php/apps/theming/js/theming?v=13 HTTP/1.1" 200 1170 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:29 +0000] "GET /nextcloud/index.php/apps/accessibility/js/accessibility?v=0 HTTP/1.1" 200 948 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:30 +0000] "GET /nextcloud/core/img/loading-dark.gif HTTP/1.1" 200 5289 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:30 +0000] "GET /nextcloud/core/img/actions/toggle.svg HTTP/1.1" 200 916 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:30 +0000] "GET /nextcloud/core/img/actions/confirm-white.svg?v=2 HTTP/1.1" 200 1013 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
172.16.94.15 - - [23/Dec/2020:11:54:30 +0000] "GET /nextcloud/index.php/apps/theming/favicon?v=13 HTTP/1.1" 200 1168 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88