"NullReferenceException" error and Managed Store stops responding

Symptoms 

In Microsoft Exchange Server 2019, the Managed Store intermittently stops responding. When this issue occurs, the mailbox that causes the issue gets quarantined, and the following event is logged:

Source: MSExchangeIS

Event ID: 1013

Details: The mailbox with mailbox guid <GUID> caused a crash or resource outage on database "<Database Name>/<GUID>".    

Resolution 

To fix this issue, install the following cumulative update: 

• Cumulative Update 15 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019