When I launch gprename, there are warnings like reading on invalid file handle. I dug into this a little and found that it was because it was trying to access /root and /ost+found and it didn't have permission.
Ah! That's why we had warnings when you opened it in console, make sense now, I never tought about permissions. I just released gprename 2.6.5 which is ok now, I just added a little if condition to make sure we have permissions, easy enough, thanks for sharing!