iCal

Can anyone tell me why I can't create or edit a calendar event on my iMac that is on a shared calendar? I am not the creator, but have read/write authority. I can edit and create on my iPhone and iPad but just not on the iMac. I keep getting a 403 error. I have MobileMe and that is hoe the calendars are shared. I tried AppleCare and went through deleting my MobileMe and deactivating it to no avail. Any help would be appreciated! Thanks!