TrueCrypt mount fails with cryptic error
Try mounting the volume as “read-only”:

TrueCrypt → Preferences → Mount Options → Mount Volumes as read-only



If that doesn't do the trick then it's possible TrueCrypt could be having some issue with file system implementation mechanism (ext4fuse) not detecting that an ext4 volume is mounted. I've used FUSE for OS X with TrueCrypt and it works without any issues, so giving that a try instead of your current mechanism might help as well.