Weird. I'm on a Mac, so this might be different for whatever platform you're running it on. But I click the Plex server icon in the menu bar, "Open Plex" which just takes you to the admin screen. It was "configured" but not signed in. So, I clicked the user bubble in the upper right, then Sign In at the bottom. This took me to the usual sign in page, which errored the first time, but succeeded the second time.

Not sure if that helps you, but hopefully it's a temporary issue for you.