No idea what's happened. I was working on the site an hour or two ago, then got called away. When I got back I'd been locked out – nothing unusual there (it's a 4.6.1 installation that's been running for about 18 months). So I hit Logout, cleared cookies, quit the browser (Iron, but the same thing's happening on all browsers) and relaunched. When I tried to log in the fields turned red and shook angrily at me: now I can't log in with any of the seven user account details.

Any ideas?