Have you tried a disk that is definitely not encrypted? This does happen, but I'm trying to remember what causes it - from memory it is copy protection (RCE?).
The ripper (CDparanoia I think is used for DVDs, too) has several special features relating to scratch detection and correction, that maybe struggling with certain copy protections. This is also what causes disks to rip much slower than your drive is capable of.
Eliminate the encryption first, just to be sure. Only other thing I can think of is maybe the 32bit version of dvdcss?