Plex server on iMac can not be accessed anymore

Hello! I am using Plex Server on my iMac since a long time and can access its content from various clients on my network without any problem so far. Unfortunately since few days, I am unable to access my Plex server from anywhere including from the iMac it is running. It looks normal with the Plex icon on the iMac top bar and when running "Open Plex ...", and select the main admin user, it is "looking for server ..." for some time and then tell me basically that I need to install Plex Media Server (which of course is already installed and running). I have reinstalled the latest version (Version 1.2.7.2987-1bef33a) and tried various things but cannot access the server anymore. When I tried to open the "Preferences..." from the Plex icon on the top menu bar of my iMac, it displays a "You must be lost. This page does not exist" message from the following URL: http://127.0.0.1:32400/web/index.html#!/settings/server/
From the web client, I do not even see the "Server" button that used to be available on the front page to change server parameter.
Can anyone help me? That would be great!!!! Thanks a lot.