In my case, TheSkyX needs to be running. It does not matter whether or not the mount is connected before ACP connects. I do (of course) have to home it, either when connecting ACP or at some other point (before or after).

If TheSkyX is not started, connecting from ACP will launch it. And then time out before connecting properly.
Thorsten, this is a timing bug in TheSky X. It's not the same thing as the 212 error.