Cannot progress past Select User on IE 11

When I log into Plex on IE on WIndows 8 (on either the desktop or the tablet mode version) I get to the Select User screen, but when I choose a user, I get the loading circle indicator which goes on indefinitely. This used to work on this PC/software configuration.

I have tried wiping the stored content from IE, I’ve uninstalled and reinstalled it, I’ve checked the firewalls, made sure the Plex server was running, etc. No joy. It works on the same PC with Chrome.

Any ideas?