I had that problem a month or so ago.
After a lot of advice from HTC support, I gave up!
I now use google calendar, I sync my PC outlook calendar with it. The phone syncs both ways to google cal.
I'm a bit of an IT idiot so wasn't keen to make the switch, but it really was easy & works very well.
HTC Sync just gave up the ghost on phone - pc sync, so I gave up on that!
Good luck.