Hi,
Should not be anything to do with permissions.
Issues like this have been fixed (so assuming you are on V6 sp2 server and client)
then the way forward is to get a set of reproducible steps and then next is to contact support. They should try and get copies of the messages you are using and see if they can repro and go from there.
We often speak to microsoft on these I think because they can tell us what's triggered the conflict.