The issue is because of mismatched cdo.dll versions or permissions.
1. Check the version of cdo.dll on both the BES and Exchange servers. If there is a lower version on the BES get the latest from the Exchange server, copy it to the system32 folder on the BES and register it. The restart the dispatcher service.
2. If it's not cdo.dll then your BESAdmin either is missing the proper permissons, is in an admin group or the permissions are set but are not working. Follow this article to check permissions: BlackBerry Search Results
This is the KB for this particular issue. Give it a read through, it's going to be one of these fixes. It always is, one way Calendar not working from BB to Outlook is always cdo.dll or the account is messed.
Let us know what fixes it.