I have the same issue with Plex and docker.
Temporally solution is to delete the related cookies but it can occur even a day later again or when restarting the docker.
Rollback and wait until Plex hopefully fix it someday?
I have running Plex in docker and there are also other docker on the same network/IP. Deleting the cookie “remember_web_*” solves it temporally.
I don’t know if it’s actual Plex fault and not another docker which corrupts this cookie and causes this issue.
I’ll keep you updated on my further testing and research.