I had the same problem, running PMS on Ubuntu 20.04 and using a Quadro P600. I could transcode using the GPU anything I played on my iPhone, however whenever I tried Plex Web the transcoder crashed. Turned out if I block uBlock Origin on app.plex.tv and/or the local IP page hardware transcoding works fine also in Plex Web.
So if you have the same problem, try turning your adblockers off.
