Another user and myself had this very conversation in PM messages late last week. He seems to have tracked it down to this:
As far as I can tell, ERPNext is making a call to this JavaScript file on cloudflare. And the page doesn’t run completely without it.
https://cdnjs.cloudflare.com/ajax/libs/socket.io/2.0.4/socket.io.slim.js
His quote here is about getting the login page to display. His observation indicates that ERPNext login screen now requires a internet connection for it to find this one file in order to complete the loading of the page.
Evidently this has changed recently and nobody is either aware of it or working on it. Maybe this is enough of a clue for you to complete the investigation and pursue a solution. I myself just do not have time to work on it since I never use ‘local only’ installations.
Hope this helps…
BKM