I have no idea how to explain what happened, but it is fixed. All I did was delete part of the URL for the dashboard and press enter to see what would happen. I then got an error “it appears the server you’re trying to access does not exist”. When I hit refresh, the Dashboard became visible, and is still visible.
I’m not sure, but I’m thinking it might have been a cookie issue?
I recently saw a version of this, and resolved it by forcing a reload in my web browser. I believe I had a mismatch between the Plex Web app version originally launched and cached in my browser and the version that was running on the server.