This is not a common error, or at least we have not had any other reports. Googling the error message doesn't give a lot of help --- the most likely problem seems to be a problem with the DVD. You might try burning the image on a new DVD maybe using a different burner and verifying the burned image (which you probably did for the original image). You can also try the standard Ubuntu 14.04 image but that should work exactly the same as the WeBWorK image.
Arnie