Symptoms
If you run the Start-ManagedFolderAssistant command while a user's online archive is in an "invalid" state, the program stops responding and generates a "4999" event in ELCAssistant.InvokeInternalAssistant and a System.NullReferenceException after seven days. This issue occurs in Microsoft Exchange Server 2019 and Microsoft Exchange Server 2016.
Resolution
To fix this issue, install the following cumulative update, as appropriate:
Cumulative Update 10 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019
Cumulative Update 21 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
References
Learn about the terminology that Microsoft uses to describe software updates.