This was pretty strange right now…
@Stefan: I tried this with a lot of apps including Business Calendar 2, Google calendar app, native app, aCalendar … everywhere it’s the same
@ben: The calendar was in the google calendar web app, but it was not shown at all… I now deleted it and imported the URL again, now it’s shown again.
The sync works, too, right now, but it could happen, that tomorrow the calendar appears twice again… so I will give you an update whether this remove-and-import-again solved the problem or just reseted it^^