Symptoms

invocation. : The EventBusInterop Container property is null.
at SourceCode.EventBus.Server.EventQueueProcessing.ExecuteAssembly(Int32 CustomEventInstanceID, Int32 CustomEventID, ResolvedPolicyMapping resolvedPolicyMapping, Boolean isConditional, Int32 eventTypeID)
at SourceCode.EventBus.Server.EventQueueProcessing.ProcessEvent(Int32 ID)
 

Diagnoses

Using the following SQL command we determined that there were multiple Server Names with the same HostName but different ports:
SELECT * FROM MK2].]HostServer].]Server]

We treat the HostName as unique, there is some logic that gets over overridden on K2 Server startup, that then fails to initialize the EventBusInterop Assemblies that Notification Emails rely on.
 

Resolution

We removed the Servers from the hK2].2HostServer].rServer] table that was no longer in use, using the following script:
DELETE
HostServer].rServer]
OUTPUT
deleted].dID],
deleted].dHostName],
deleted].dHostPort],
deleted].dRunning],
deleted].dLastUpdate]
WHERE
HostPort] = OLD_PORT_NUMBER

Email notifications were send after a K2HostServer restart.