Highly recommended fast seller response. Just tested it last night too and works great.
Update
I have found now that not every single udemy course is downloadable. Using this script.


Seems like this problem is common when you download too many videos. Mine stop working after 12gbs of content however the script still downloads free content. Also if I move the udemy_ripper.exe to another computer it will start working again. On github I've seen the same issue if you download too many videos. You could reset the script but I wouldn't know how to do that since I have no knowledge of Python language. People that were using the version on github without the GUI were able to reset the problem and continue downloading videos. OP if you could explain what to do when you get "cannot download video" error what should we do?