SyncEventOrClientNotFound
The desired event was not found when fetching pending events. This can happen when this client has been offline for a long period of time, and the backend has deleted old events.
This is a recoverable error, the client should:
Do a full slow sync
Try incremental sync again using the oldest event ID available in the backend
Warn the user that some events might have been missed.
This could also mean that the client was deleted. In this case, SlowSync will fail. The client should identify this scenario through other means and logout.