When using the local web client I have a mystery library that has appeared. I have changed a number of libraries around, so this is probably something I have done in the past. I have attempted to unpin the library but it does not do anything. When I click on the library the web page states “you must be lost”.
The mystery library does not appear if I use app.plex.tv
Wow. I think if it doesn’t happen in the hosted web app (which is newer), then you might have stumbled upon a rare bug which is already fixed.
So you only need to wait for a newer version to get included with a new server version. Or use the hosted app instead.
You could also try resetting the whole home page and the side bar to their defaults. But this will also revert any customizations you might have done so far. https://support.plex.tv/articles/customizing-plex-web/
(scroll all the way down)