I had the same problem with one of my users today... just got off of blackberry technical support. You more than likely have a corrupt "Message" database.
Here is what you need to do.
1. Open up desktop manager and open Backup and Restore.
2. Do a complete backup of your device.
3. Once complete, go back into backup and restore and click the advanced button.
4. Scroll down the device databases column on the right until you get to messages, highlight it and click the "left" arrow button.
5. If your "Messages" database is corrupt, you will receive an error message.
6. If you receive the error message, then highlight "Messages" and press the clear button.
7. Acknowledge the warning message and click close.
8. Wipe the handheld, ensuring that "Content Protection" is disabled.
9. Restore the backup you did, but in the "Desktop File Databases" on the left, include everything but the "Messages" database.
10. Once complete, you may have to re-send service books, or re-activate if you are using a BES. (Depending on if you get a "Decryption failure".
Worked for me, hope it helps you out.