Crowd Sync removes users instead of adding/updating them

How to reproduce:
1. Integrate a Fisheye and a Crowd instance. Add a few users to Crowd and make them available to the FE App.
2. Run the Sync in FE. Nothing will happen (no users will be synced).
3. Login to FE using a Crowd user. The authentication works and the user is added to the User list in the Admin console
4. Run the Sync. The Crowd user will be removed from the User list in the Admin console. In the database it will have its name appended with "_old".

Instead of adding new users to FE or just updating them, the Sync removes all Crowd users.

The FE and Crowd logs don't show any error. When logging the REST messages between FE and Crowd, all calls end with "200" and the users are provided by Crowd correctly when a sync is run:

"<users expand="user"><user name="user1"><link rel="self" href="http://nastro-azzuro:8095/crowd/rest/usermanagement/1/user?username=user1"/></user><user name="user2"><link rel="self" href="http://nastro-azzuro:8095/crowd/rest/usermanagement/1/user?username=user2"/></user></users>"