What does this mean and is it a critical problem?
this is not a critical problem. This is a safety feature.
During a sync cycle Allocatus loads all your calendar events from Microsoft Exchange to compare them to the data that Allocatus has in the database. We have experienced that Exchange sometimes does not deliver all of the items in an users calendar due to a problem in Exchange. If Allocatus gets less or more events as Exchange claims to have, we stop the synchronization as this could lead to unwanted behavior in Allocatus. E.g. Allocatus could recognize calendar events as deleted and re-add them to the calendar and then they are in the calendar twice.
In the next synchronization Allocatus will synchronize this user correct again.