Just an extra step that may or may not help (test this on your own before a director, obviously). But set the PIM back to MS Outlook, then go into Backup/Restore > Advanced and clear the Calendar database (to remove all appointments). Then set the PIM back to BlackBerry Wireless Sync and do a Synchronize Now to enable it (I think thats what you had to do with 3.6). Just make sure the calendar appointments are populating back to the handheld.
Again, that may do absolutely nothing aside from 5 minutes worth of your time wasted, so its up to you.