Hmm. I had that same problem once before, and for the life of me I cannot remember what I did to fix it. I wanna say that I deleted everything dealing with TiBu from my SD (and backed up the backup file to my pc), but I'm not entirely sure.
You could try booting into recovery and fixing permissions to see if that helps.
If that fails, then try;
Uninstall TiBu (both the regular TiBu app and it's pro key app), then backup the TiBu folder to your PC, then delete that folder from your internal SD. Make a backup also of data/com.keramidas.TitaniumBackup to your PC, then delete it from your internal SD. Reboot the phone.
If that fails, I recommend contacting the TiBu dev. If you do, be absolutely sure you are very specific in each and every troubleshooting step that you tried. This will help him better assist you.