Had the same bb (7100i) and the same desktop computer for 16 months, and suddenly today it starts giving me an "unable to complete request" error during the "processing handheld calendar" portion of the Intellisync process.
I use a mini USB cable to sync and have not had problems.
If you use intellisynch you will need to cancel it prior to the crash. and then follow instructions below. If your phone doesn't automatically start to sych when connected the just follow the instructions below.
Using Desktop Manager go into backup/restore>Advanced. Then clear the Calendar on the Hand Held. Say Ok and then kick off the synch.