There are no add-ons installed. I tried with new, virgin Chrome and Firefox browser profiles, with all default settings. The problem is unchanged. It seems like a server side issue.
There were add-ons in the browser I took the screenshot from , but as I stated, I tried separately in new browser profiles, and gotten the exact same result. Here are the screenshots for those.
I am having the same issue and i noticed that the URL had a port of 8080 on it in the browser. Once i removed the :8080 It connected fine. I am not sure yet where the port of 8080 is coming from. also, if notice that i can connect to settings from the drop down in the upper right and it will connect. then i can click the home icon in the upper left and get to my home page. This method also does not append the port :8080. Maybe some plex redirect? i have this link on multiple computers and multiple browsers. I have also typed in plex.tv, put in my user name and pass then clicked the open Plex button and it times out with the app.plex.tv:8080 URL. Again if i remove the :8080 it will connect.