Microsoft has released security updates (SUs) that resolve vulnerabilities found in:
Exchange Server 2013
Exchange Server 2016
Exchange Server 2019
IMPORTANT: When manually installing SUs...
Updated Mar 16, 2022
Version 6.0
Blog Post
Like many of you, I had a client install this update and it went belly up. 5 Microsoft technicians and 36 hours later, we finally got the server back online and operational.
The symptoms: The ECP redirecting to OWA in a endless loop causing a 404 error and mail flow problems. The published workaround for the OWA issue to run the UpdateCas.ps1 and UpdateConfigFiles.ps1 Powershell Scripts did not fix the issue. It persisted afterwards. Tried uninstalling the Security Update and it made no difference. ECP and OWA didn't work regardless what was tried. Microsoft re-installed Exchange 2016 CU22, searched for missing DLL's, and fixed some file system permission issues, but that didn't even make a difference. After countless hours of searching and trial and error, I noticed that the OWA showed that the FrontEnd/Http/owa/Auth/Login.aspx file was missing. I checked the physical directory and sure enough, the files in that folder were missing. I restored from a Previous Version backup prior to the Security Update installation and voila! ECP and OWA was working again.
As for mail flow, that issue had to do with the Microsoft Exchange Transport Submission service being missing from Services. I'm not sure how that happened either. Using some Powershell commands to attach a SSL certificate to a Receive Connector seemed to have restored it. Once all the relating transport services were running, mail started to flow again.
Once these issues were corrected, we re-downloaded the Security Update and re-applied it. The second installation installed without issue.
I hope that this information helps others out there and save you some time in troubleshooting.