so.....
reinstalled plex yesterday (delete image + container, deleted appdata/plex and all it's contents, install from scratch).
worked fine for about 1/2 a day, then froze. restart the docker, worked for a few hours. came back again tonight and it's frozen a 2nd time, restarting give the error above.
no problems seen with unraid or the other dockers, everything seems to be fine.