Youtube seems to be doing some A/B testing with the comment system which has made proxies like Invidious and yt-dlp/Newpipe unable to load comments. There is a patch for Invidious [1] which solves this problem but it is not in master yet. I tested it on my own instance and it does solve the problem.

[1] https://github.com/iv-org/invidious/pull/4576