(Like many of my technology postings, this is really here for my own notes on problems I run into, the solutions I try and what works when. They are updated as I learn.)
Plex is great to organize digital storage.
Suddenly though all of my devices would given me the above warning when I tried to access the server. I made sure to run updates first by right clicking the Plex icon in the lower right corner and selecting "Check for Updates". I then went into Task Manager and shut Plex down.
Click on Cortana or do the "windows +R" or whatever you use to type in regedit and get into the registry. We are going to "
Computer\HKEY_CURRENT_USER\Software\Plex, Inc.\Plex Media Server" and delete the values for
Start up the Plex server again, maybe reinstall the client. Just restarting it worked for me.