Is BPS installed on its own server or on exchange?
What version exchange?
What version CDO.DLL is installed on BES and exchange?
KB04912 - How to check if and where the CDO.dll file is registered on the BlackBerry Enterprise Server View Document
Use search on Exchange to find it and dont forget to look at hidden files during your search.
KB05270 - How to troubleshoot one-way wireless calendar synchronization issues View Document