Clarify `trusted_domains` setting

          > 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 https://github.com/nextcloud/server/issues/46381#issuecomment-2223538864

Open the linked Nextcloud Admin Manual “Trusted Domains” configuration section and review its guidance for virtual hosting. Clarify that only the proper domain should be listed when virtual hosts share an Apache server, including that default entries such as localhost and 127.0.0.1 may need to be removed. Done when the warning is explicit and matches the reported configuration behavior.