> It does, however, run a check against each of your configured `trusted_domains` [...]

If one of the listed domains returns a 200 code, that error will appear. [...]
Usually this is when people discover they have many unnecessary domains in their config [...]

You were right. That was the culprit.

I am using virtual hosting with several sub-domains on a single Apache server with the same IP address. One sub-domain is cloud.my-domain.tld (for NC), photos.my-domain.tld (for Lychee) and www.my-domain.tld (which only serves an imprint and other required legal information).

I had the following entries for trusted_domains:

'trusted_domains' => array (
  0 => 'localhost',
  1 => '127.0.0.1',
  2 => '[::1]',
  3 => '<my-public-IPv4-address>',
  4 => '<my-public-IPv6-address>',
  5 => 'cloud.my-domain.tld',
),

localhost and 127.0.0.1 had been listed by default and I added [::1] as well as my public IP addresses. However, if a client connected to any of those, the Apache server redirect the request to the default sub-domain www.my-domain.tld which serves the imprint. I removed every entry but the last one. Now the error disappeared again.

Maybe there should be an explicit hint at NC Admin Manual: Configuration Parameters - Trusted Domains that for virtual hosting only the proper domain should be listed. In particular, with virtual hosting the default entries localhost and 127.0.0.1 must be removed.

Originally posted by @nagmat84 in nextcloud/server#46381 (comment)