Maybe a problem with Microsoft Defender signature update. Goto WSUS console and check the what patches were approved last week. If you’re lucky, the corpus delicti shows an error. Otherwise check the patch statistics of a client machine, check machines report in WSUS. Revoke the problematic one.
It it doesn’t help, try this:
%PROGRAMFILES%\Windows Defender\MPCMDRUN.exe" -RemoveDefinitions -All "%PROGRAMFILES%\Windows Defender\MPCMDRUN.exe" -SignatureUpdate