The following solution worked for me, but reading through this post, it might be a hit and miss situation.
I changed WebUI under advanced settings to https://app.plex.tv/desktop. That fixed the problem for me.
FYI - binhex, I tried your solution above and it worked temporally which was very odd, then I was back to square one.
PS Thanks for all the hard work you're putting in to these Dockers. I don't want you to think the effort has gone unnoticed.
Regards,
MrFryRye