Try disabling IPv6 in the settings of Plex server. (stop and restart the server afterwards)
Your log shows access by IPv6 during the failing configuration attempt.
Do also try to temporarily downgrade your server software.
You should be able to find older software versions in the /Updates subfolder of your Plex data folder.
Or see if the new beta can help with this issue: Plex Media Server - #558 by janoskk
