The reason is ticking this checkbox:
It does not only remove the login tokens of all Plex clients, but also that of all servers.
Plex server is an, err, server. Not some local app for categorizing media files, but an actual server.
As such, it has some stricter security requirements as you may be expecting. One of these is the policy to require authentication from anyone who tries to view or mange the media collection.
And since the user accounts don’t live on your local server machine, but on plex.tv, it means that the server also has to be logged into an account before anyone can access it.
To recover from this, perform the procedure in this post: You need to reclaim your server, but don't dare to touch the Windows Registry
After doing so, use this method to re-claim your server back into your account: New Claiming Requirement for Windows
