I did some testing and here are my findings:

• it only happens to new videos (old videos from before this issue started happening, are fine)
• it only happens when trying to download 720p@24/30fps h.264 videos (basically pre-DASH format used for old, pre-HTML5 browsers, probably other non-DASH formats are affected too)
• affected videos have 720p@60fps h.264 variant with really low size (terrible quality)
• DASH variants for affected videos work fine (I tested both 720p and 1080p, 30 and 60 fps, h.264 and VP9 HQ)

I noticed that it happens more often - few months ago I had this issue with only one channel and not always. Now all new videos from this channel are affected and it happens to other channels too.

It seems that YT is phasing out support for old browsers with no DASH support. This is sad because it offered best quality for 720p videos and download speed wasn't capped like for DASH videos. Also, this format was the most widely supported by "old" players, devices and TVs.

Suggestion for affected people: do not download 720p@30fps h.264 videos (possibly some 480p and 360p too) - change your default codec to VP9. If you player/device can't support it, you can download 1080p h.264 since it supports DASH.

Suggestion for jDownloader: change default format for 720p@30fps h.264 (and possibly all affected variants) to DASH-supported version since more and more videos won't work. Leave this variant as an option in the plugin (since old videos will still work), while DASH version is default. Also, it would be nice to know which variants are DASH and which are not (all 1080p, VP9 and 60 fps videos are DASH - not sure which variant are non-DASH, except 720p@30fps h.264).