Start-ManagedFolderAssistant causes a "4999" crash in ELCAssistant.InvokeInternalAssistant with System.NullReferenceException

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.

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