Your BlackBerry Messenger database is corrupt.
Connect the device to your PC and open the BlackBerry Desktop Manager.
Open the Backup/Restore icon.
Select the Advanced button.
On the right side, you will see the databases of your BlackBerry.
Select BlackBerry Messenger, and click Clear.
Confirm that you want to delete your BlackBerry Messenger list.
After this is done, you will have to create your list again, so remember the PIN #s on your list if it's more than just yourself.
If you're one that has to be safe about everything, perform a normal "Backup" in the Backup/Restore menu, before going into Advanced.