Released: April 2021 Exchange Server Security Updates

Hello,

I tried this update on Exchange Server 2019 with the latest CU and it worked just fine, but unfortunately..

I applied the patch for Exchanger Server 2016 with CU19 installed on a Windows Server 2012 R2, but the server broke:

- All Exchange sevices seem to be running, but no one can connect to the Exchange server with their Outlook clients

- The ECP/OWA logins are "successful but show only a blank page

- Starting the Exchange Management Shell gives a lot of error messages and can not find any exchange server(s) on any site. Parts of the error messages:

VERBOSE: Connecting to bla.blaaa.net.
New-PSSession : [bla.blaaa.net] Connecting to remote server bla.blaaa.net failed with the follo
wing error message : [ClientAccessServer=BLA,BackEndServer=ebla.blaaa.net,RequestId=aff1b744-4ccd
-434c-ba90-29bec457cc17,TimeStamp=2021-04-14 16:25:16] [FailureCategory=Cafe-SendFailure] For more information, see th
e about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Microsoft.Excha ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
gTransportException
+ FullyQualifiedErrorId : -2144108477,PSSessionOpenFailed
WARNING: No Exchange servers are available in the Active Directory site my-site. Connecting to an Exchange server in
another Active Directory site.

VERBOSE: Connecting to bla.blaaa.net.
New-PSSession : [bla.blaaa.net] Connecting to remote server bla.blaaa.net failed with the follo
wing error message : [ClientAccessServer=BLA,BackEndServer=bla.blaaa.net,RequestId=33fd0e97-f88e
-4a91-a6d6-e7bcaae62ee5,TimeStamp=2021-04-14 16:25:26] [FailureCategory=Cafe-SendFailure] For more information, see th
e about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Microsoft.Excha ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
gTransportException
+ FullyQualifiedErrorId : -2144108477,PSSessionOpenFailed
Failed to connect to an Exchange server in the current site.

The installation did not encounter any problems at all and went "fine".

Any clues?