Edit:
Found it!
I have found the solution here:
https://help.nextcloud.com/t/s…c-client-on-linux/47549/4
I have added these lines to my config.php file:
'overwrite.cli.url' => 'https://nextcloud.domainname.dnsprovider.org',
'overwriteprotocol' => 'https',
'overwritehost' => 'nextcloud.domainname.dnsprovider.org',
'trusted_proxies' =>
array (
0 => '1.2.3.4',
),
And it is loading now.
It is working also in both configurations:
'trusted_domains' =>
array (
0 => '1.2.3.4:1234',
1 => 'nextcloud.domainname.dnsprovider.org',
),
and:
'trusted_domains' =>
array (
0 => '1.2.3.4:1234',
),
and also without this variable:
'trusted_proxies' =>
array (
0 => '1.2.3.4',
),
Not sure which configuration is 100% correct (or which entires are 100% required) but at least it is working, also on my Android phone