Released: April 2021 Exchange Server Security Updates

After Exchange 2016 CU20 on the other server /ecp and Exchange Management Shell still worked on the second server.

After installing the patch it broke on that server too.

/owa and /ecp state 'Cannot serialize context' whilst PowerShell throws:

New-PSSession : [SERVER2.domain.local] Connecting to remote server SERVER2 failed with the following error
message : For more information, see the about_Remote_Troubleshooting Help topic.
At line:1 char:1
+ New-PSSession -ConnectionURI "$connectionUri" -ConfigurationName Micr ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OpenError: (System.Manageme....RemoteRunspace:RemoteRunspace) [New-PSSession], PSRemotin
gTransportException
+ FullyQualifiedErrorId : -2144108477,PSSessionOpenFailed

For both the 2016 servers. It connects to the Exchange 2010 that's still alive after that.

Back-end certificates look fine on both servers.