Plex Server Windows can't add any library

Is your ISP xfinity? Xfinity XFi Security

Or does your router perform ‘DNS rebinding protection’?

Does your default web browser have 3rd-party add-ins?
Try using a different browser (or disable all add-ins and use the ‘incognito/privacy’ mode) and load this address http://127.0.0.1:32400/web