The problem is the encryption on the disks. Google a program called "Dvd43" and install it. It's a tiny program that runs at startup and decrypts the DVD as soon as you put it in the drive!
Once I installed it, I've not had a problem since.
On a related note, there's a few Handbrake settings that I have come across that work for me without fail.
1) Pick the "iPhone" setting and change the following:
a) change to MPEG4
b) select "auto crop"
c) uncheck the chapter markers box
I leave everything else the way it is and get good quality videos at reasonable sizes.
Enjoy!
Found the program at:
http://www.dvd43.com