The BB Creates a calendar for each Email address you have.. I had a similiar problem when I removed all my Gmail accounts and re-added them due to the BIS 2.6 upgrade..
My default calendar was
so that is what I synced with.. after I removed that account from my BB all my calendar events for
got transfered to "Device Default" and I could not sync it with my Desktop.
What I did was:
1. Make sure you desktop calendar has all the most updated info in it.
2. I deleted all my calendar entries from my BB, by using the Backup/Restore -> Advanced and cleared my calendar. From the Desktop Manager. (You might want to back it up first but that is your choice.)
3. On the BlackBerry Option -> Advanced Options -> Default Services, CICAL is the default calendar setting set that what you want.
4.Then sync with your desktop and it will add all your calendar events back in to the calendar you want.
Then when you open you calendar you will see all your events on the first screen.