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It shows "here was an error communicating with your Plex Server. Check the logs and verify your server connection in the settings." And the logs show "Tautulli Pmsconnect :: Unable to parse XML for get_local_server_name: 'NoneType' object has no attribute 'getElementsByTagName'."
I have just installed tautulli and have no connection problems with the browser. With the iOS app there is a security problem concerning the connection. But it works in the browser without any problem.
Plex IP or hostname: IP of your plex docker container
Plex port: 32400
Uncheck remote server
uncheck Use SSL
Plex Server URL: http://IP_Of_Docker_Container:32400
Plex weburl: either plex.tv url or your own plex domain name
Also make sure you get the Access token at the bottom of the settings page. All of that worked like a charm for me.