Desktop client does not sync, mobile client does

Hi together,

Nextcloud version (eg, 20.0.5): '
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): nginx/1.18.0 (Ubuntu)
PHP version (eg, 7.4): 7.4

VPS ssh forward proxy to homeserver, on homeserver using nginx to handel TLS and nextcloud access etc. Using self-signed certs.

The issue you are facing:
The service is accessible with the mobile clients and also syncable, but the Nextcloud desktop sync client on Ubuntu is throwing errors and I am unable to sync/upload.
I am able to connect to the nextcloud instance but I am unable to sync to it:

Screenshot from 2021-03-24 14-32-54

Judging from the logs, I would assume especially the last two entries are relevant, but I do not know what to do.
Also it tells me that appinfo/app.php is deprecated, not sure why.
The desktop client is the one available in snap store, but I also tried unsuccessfully the provided AppImage.

Any help is appreciated.

Is this the first time you’ve seen this error? (Y/N): Y

The output of your Nextcloud log in Admin > Logging:

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'passwordsalt' => 'QWwiJCFuAc1RwJJAszFY8nYmlOOfGt',
  'secret' => 'Sa3Sr4gNJ3ta123Idq9VEgtpsWD3nwEV7BEdIIUSzqfBYrfr',
  'trusted_domains' => 
  array (
	  0 => 'localhost',
	  1 => 'XXXXXXXXXX',
  'datadirectory' => '/var/www/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '',
  'overwrite.cli.url' => 'http://localhost',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'XXXXXXXXXXX',
  'dbpassword' => 'XXXXXXXXXXX',
  'installed' => true,
  'instanceid' => 'XXXXXXXXX',
  "log_type" => "syslog",
  "logfile" => "",
  "loglevel" => 0,

The output of your Apache/nginx/system log in /var/log/nginx/{access|error}.log: - me [24/Mar/2021:14:38:03 +0100] "GET /ocs/v2.php/apps/notifications/api/v2/notifications?format=json HTTP/1.1" 304 0 "-" "Mozilla/5.0 (Linux) mirall/2.6.4-1 (Nextcloud)"