I'm afraid that your trophy data got corrupted.
The only way to fix this is to delete the trophy folder from the user via FTP, rebuild the database, and relaunch the game to reinstall the trophy set.
I don't remember if it was needed to delete the game installation data.
Another more "destructive" way to fix this is to delete your local user from the PS3. But be careful, as this will remove all content attached to this local user.