Since my Peertube instance was down for a few weeks I am not entirely sure when this happened, but I suspect something in Peertube version 8.2.x changed regarding the ffmpeg settings that broke hardware accelerated video transcoding.

It still works for VOD and livestreams work with the default CPU transcoding.

I suspect the plugin I use became somehow incompatible, but if anyone here has similar issues that might narrow it down further.