If you use Freetube, you haven’t failed to notice that Google is on the war path again and broke it a few days ago. See this bug report.
Please note that it’s been fixed in this PR. If you download the nightly build #5652, it now works like a champ.
Thanks team Freetube!
If this happens again (which is very likely, I fear):
I’ve been downloading videos with
https://flathub.org/apps/org.nickvision.tubeconverter (yt-dlp gui)
and watching them with mpv. Still looking for a Tube-Archivist/Pinchflat stlye app that doesn’t download the whole channel (or makes me paste single URLs) but has a nice Freetube-like interface for downloading and watching offline …