A list of the groups could not be retrieved in GroupCalendar Web Part

When i try to add a GroupCalendar Web Part to a modern Sharepoint site I only get this message:

A list of the groups could not be retrieved.

The message is translated from the German version. Sorry don't have a English version at hand. And in the Web Part it says: "You need to join the group"

I tested different scenarios with different users. It does not wort even when I'm the owner and also a member of the Group. When I Click on the link in the WebPart it say "You already joined this group"

What am I doing worng? Did something change in the Sharepoint universe that i have missed?

We are only using Sharepoint Online, no on prem Server.

AD is synced with Azure. But for this use case with M365 Groups that should not be the problem as they are created in the Admin Center. 

I am at a loss. Any help is appreciated.

Regards