Hmm. Normally if you have a copy-protected DVD, it will tell you before it will even do anything, but perhaps you have a DVD with strange copy-protection... it's really hard to say.
If you are doing a 1:1 backup, you might want to download DVD Fab Decrypter or some other DVD backup application that doesn't care about any copy-protection that's put in place. Once it's ripped to your hard drive, you could use Nero to make a video DVD, using that folder. I've had to do this a few times in the past.
You might also want to try copying the DVD to the hard drive with Nero as an image, if you are able to do so with the Windows version. You can with the Linux version, so I'm assuming it's the same.
If you don't get anywhere, let us know and we'll figure it out. I'm doubtful it's the application though, but it wouldn't hurt to re-install anyway just in case.