Thanks @pfaffman and @pmusaraj it worked now. And I got an idea what’s going on here (with all instances because same configuration)…
"Registration failed: 400\nError: {\"errors\":[\"You supplied invalid parameters to the request: Domain validation failed: Failed to fetch https://forum.concarne.org/.well-known/discourse-id-challenge with error code: 404\"],\"error_type\":\"invalid_parameters\"}"
As said, I have Discourse behind an Apache webserver. For the Letsencrypt certificate challenge I have an exception for queries to the .well-known directory.
Brb, I’m trying to change that.