Before trying the above, do this:
Settings - Server - General
Sign out
Then log out the web app as well
(use the user avatar/initials in the upper, right corner and then “Sign Out”).
Delete the cookies of plex.tv from your web browser and clear the browser cache.
Then load the local web app from your server (http://ip-of-your-server:32400/web) and log into the correct plex account.
Let it sit for a few minutes. It should eventually notify you of an unclaimed server in the local network. Claim it.
These instructions only apply if your server is in the same local network as your web browser.
