The managed domain is experiencing a network error - can't find the problem

Question

Hi, we got the following error for our AZ AD Domain Services:

Severity Critical
ID AADDS104
Raised Tue, Jan 19, 2021, 07:56:44 UTC
Last detected Tue, Jan 19, 2021, 07:56:45 UTC

Diagnostics for Domain Services:
Diagnostics completed successfully at 20.1.2021, 09:24:41.
Validation
Result West Europe/azure-adds-subnet OK
DNS records OK

Monitors
Backup Last backed up on Fri, 15 Jan 2021 08:30:46 UTC
Synchronization with Azure AD Synchronized on Wed, 20 Jan 2021 07:56:51 UTC.

I checked the Inbound and Outbound security rules already:
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/alert-nsg#aadds104-network-error

Why does the health status not change and stays in the "needs attention status" ?

We made no changes as well. So how can we find the problem ?

Thanks
Philipp