When trying to rip a DVD, it freezes after 30 minutes.
When I try to transcode a small part of the dvd, I get this error.
That might be a bug in the software. Have you tried using a different application? I use Handbrake and have only had good experiences so far. (They don't have a dev for 12.04 yet, but the 11.10 version works perfectly on 12.04.)
If your DVD is copy protected Handbrake will most likely also fail. To still make a copy of your DVD you could try this slightly more complicated method.
It would be nicer if you write the name of the app that you are using.
Anyway use Handbrake, this application is modern and supports a much more efficient and scalable codec, Handbrake also support the old AVI container for some legacy use like ripping a file for playing it in your video player that only accept AVI files.