Ok, I just tried something else: I responded to a post of my account on mastodon.online and it worked!
I wonder why. Might mastodon.social and mastodon.online behave differently?
Or did it work, because mastodon.online already knew about @angelo@angelo.veltens.org, because I did the search there before, but I did not do that on mastodon.social (due lack of account there).
Lucky that it worked, but still confused 