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.
The logs are not very helpful other than confirming that it is a ffmpeg error: exit with code 234 Conversion failed, which I can’t find anything useful online about.
Edit: ah, it also gives a super long error from ffmpeg itself. Still trying to parse it.