I think the old rippers suddenly stopped working, as if Udemy patched it. The only one working right now is the Ultimate Ripper but it can only download videos for 5 minutes(preview time).

The links are still working and safe. BUT the Ultimate Ripper, you might want to be careful with that. My Bitdefender reported potentially malicious but I still use it.

The Ultimate Ripper works for 5 minutes because of the preview mode.

Is there a way to extend/stop the preview timer?

I have a problem with the Original Udemy Ripper, when I tried to download a course, it simply says "cannot download" again and again. Is there any fix for this?