Federation NC 29/30: "Could not detect any host. Error while syncing ..."

[nagmat84]

I am running a single server with two virtual domains for two different NC instances, e.g. https://cloud.my-domain-1.tld/ and https://cloud.my-domain-2.tld/ Mutual, federation is set up.

The logs for domain 1 contain the following errors (always as pairs):

Error    fed-sync      LocalServerException Could not detect any host
                       Error while syncing https://cloud.my-domain-2.
Error    federation    LocalServerException Could not detect any host
                       Server sync for https://cloud.my-domain-2 failed.

The logs for domain 2 contain show the same errors but refer to domain 1, of course.

What triggers these error? How do I solve them?

Some (possibly useful) facts about the setup:

• Both instances have both domains listed in trusted_domains in their respective config.php. IP addresses or localhost are not listed.
• The forward DNS entry (A/AAA) for the public IP is server.my-main-domain.tld, the reverse DNS entry (PTR) for the public IP resolves to server.my-main-domain.tld.
• The domains cloud.my-domain-1.tld and cloud.my-domain-2.tld are CNAME entries and point to server.my-main-domain.tld.