I think when you "dismiss" an appointment and Sync, Desktop Manager will delete it. All I did was set an option to prompt before changes and skip any deletions when I sync.
try this, calendar>menu>options. see how many calendars you have listed. and see if any of them show any entries.
also calendar>menu>select calendar> set to show all.
also, from the main menu, options> advanced options> default services> select the calander you want as default.
i had same problem. had nothing to do with memory. i had conflicting calendars and or a corrupt calendar file. i dont know how to tell you to fix it. just what i did. if all else fails id call your provider or call BB. they walked me through a wipe of the phone and resetup the "default" calendar. anyway, it was confusing but the problem is solved now. But make sure you back up your phone if you end up needing to wipe it.