For anyone else using this docker and getting a "Unable to authenticate." message when signing in using Plex: there is a bug in the current version of Overseer itself (v1.33) which is causing this.
https://github.com/sct/overseerr/issues/3507
Downgrading to v1.32.5 fixes this issue, to do this in Unraid simply edit the binhex-overseer docker container and replace Repository value with "binhex/arch-overseerr:1.32.5-1-01" and it will roll back to the previous working binhex version.