I don’t know if this is the place to report bugs but I don’t see bug page, so here goes. For awhile now Plex web has started hanging when I rewind something that I’m watching. Hitting the left arrow key 2 or 3 times alone is usually enough to do it. Very occassionally it’ll recover after spinning for several seconds but usually the only way to fix it is to stop the video I’m watching and restart it. I usually use Firefox but this happens with Chrome too. I did find a post from 2013 or so that identified this as a bug and it looks like it’s back.
I’m currently using Plex server version 1.22.0.4163 (Beta channel) and web player 4.51.3 but it’s been doing this through several upgrades now. My PMS is running on Ubuntu 16.04.7 if it matters.