After updating Exchange 2019 to CU9, ECP returns "500 Unexpected Error"

Windows Server 2019.
Exchange Server was 2019 CU8, updated to CU9.  Did not have any issues during installation.

However the /ecp (Exchange Admin Panel) no longer works.  Minimal info is shown in browser:
"500                Unexpected Error :(                                    An error occurred and your request couldn't be completed. Please try again."

All services seem to start normally after restart.  OWA still works.  Outlook client access works.  Email is still flowing in/out.

sfc /scannow and dism /restorehealth etc. both come back clean, no problems reported.

Help!