Can't access the server from the server, but can from a different machine

[OttoKerner]

1. Don’t use Edge as the web browser
2. Make sure that the web browser is logged into the same plex-tv account on every machine. It must be the same plex.tv account which is also used by the server.

see also

Plex server can not be found on PC it is installed on, but is accessible from other devices

Your web browsers are using a different plex.tv account than your server and your iOS device. You probably used ‘Login with Google’ Don’t. Logout and use ‘login with email’ supply the right email address and Plex password. If you cannot remember which email you used to set up the server, take a look into the Registry. The key PlexOnlineMail should contain a hint.