We have exactly the same problem.
As soon as we click on mxmContacts type object as an authenticated user you can see "Insufficient privileges".
If you try access as anonymous user you can see its contents (but then there are other problems, like empty groups and invisible contact details (you have to mark it using a mouse, sometimes a few times, to make it permanently visible)).
The other problem is that when you add the second (EMPTY!) mxmContacts object you can see contacts and groups belonging to the first one.
As soon as we click on mxmContacts type object as an authenticated user you can see "Insufficient privileges".
If you try access as anonymous user you can see its contents (but then there are other problems, like empty groups and invisible contact details (you have to mark it using a mouse, sometimes a few times, to make it permanently visible)).
The other problem is that when you add the second (EMPTY!) mxmContacts object you can see contacts and groups belonging to the first one.