I have the exact same problem
I have done what CBR929 suggests, but it doesn't make any difference. Never works. Haven't been able to use DVDFab to convert video files for many months (I re-try after every DVDFab update). I am able to rip DVDs without any problems (which to my layman's mind should be more difficult to do that converting video files, but I really have no idea). I, however, have had no problems using an alternative video converting program for the same thing. I've paid a lot of money for the lifetime registration for DVDFab, though, and I'm very disappointed this issue hasn't been resolved in all that time.