Is this effecting all users?

If so, check the versions of:

MAPI32.dll and CDO.dll on both the BES and the Exchange

These versions must be matching. If there is a mis-match with either of these files then this may be causing the problem.
Also check permissions on the BB Service Account and possibley re-register the CDO.dll file.

If is only effecting 1 user, then delete and then undelete the Desktop [SYNC] service book. This will force a slow sync which may also resolve the issue