Here is a Message from the Log Analytics:
Failed to add user = ≤AzureAD\xxx@X .com≥ to group = Remote Desktop Users. Reason: Win32.ERROR_NO_SUCH_MEMBER 1
Another Issue i recently discoverd: The VM which is listed in the hostpool is NOT AAD joined. The VM is simply not available in the AAD Devices list.
Although, the Detail View of the Session Host list shows status "available" with a DomainJoinedCheck successfull.
I think the main reason is that the VM is not AAD joined.
But why is that?