Sometimes sync fails for a user without indicating a cause. On the Activity Sync log, the System Administrator can see the error: <Reason = keypath Mobile_ID_vod__c not found in entity <NSSQLEntity Time_Off_Territory_vod__c id=132>.
This message means that a particular field is no longer present on an object's table within the device's database.
The root cause is due to lack of visibility to a field. The most common scenario is that the Field Level Security (FLS) for the user is removed between syncs.
The way to identify the field is written on the message:
<Reason = keypath [FIELD NAME] not found in entity <NSSQLEntity [OBJECT NAME] id=132>
Provide access back to the affected field to solve this issue.
CRM Help Documentation: Sync Overview