Microsoft has released security updates for vulnerabilities found in:
Exchange Server 2013
Exchange Server 2016
Exchange Server 2019
These updates are available for the following specific...
Updated May 04, 2021
Version 8.0
Blog Post
Perhaps something went wrong with the web part.
Did you run from evelated prompt like alex kim said? Otherwise run it again from elevated prompt and if it doesn't work then there's a UpdateCAS.ps1 in the bin folder of the exchange installation. Try running that and see if things improve. You can run it from regular (admin) powershell.
Yes, multiple environments without issues, all of them after renaming accounts with $ in them and fixing powershell I broke whilst troubleshooting. The environment with issues I removed and recreated powershell VDs, oddly those servers showed a VD for powershell on both the front-end and the back-end. Removing the back-end one broke some things in IIS I presume (application pool was gone and the 'powershell' on back-end showed up as a folder instead of an application). Threw the same error code in Management Shell as with $ accounts. After recreating the IIS config manually it started working (w/o $ in account then, still doesn't with $). It's not related to the account not having a mailbox.
GregS410 I thought issue might stem from some non-default config on the powershell VDs (mostly them appearing on back-end too - someone might have created them by accident). If it comes from me, I'd let it be, our issues came solely from $ in account.