Last week there was an rpi-update which introduced a bug in chromium video playback.
I have 2 Pi 4 systems setup (one at work and one at home). I did the rpi-update on both systems, and one stopped playing youtubes in chromium and the other one didn't. I reverted to the previous rpi-update and the problem disappeared. That is an odd bug!
It turned out that the only difference between the two setups was that the working one used HDMI audio and the one that broke used analog audio. I tested this and while playing a youtube in chromium, setting the audio to analog freezes the playback, changing back to HDMI resumes playback. There seems to be no affect on any other graphics program apart from chromium videos.
The developers probably know about this, but since I couldn't find any mention of it, i thought it might save some confusion for people who like to live dangerously with rpi-update.