UAC elevation prompt doesn't show password prompt. Why?

Windows 7; I've got UAC turned on, but I'm not logged in as an administrator. This means that when I click 'Run as Administrator', I'm prompted for the password for the Administrator account.

Until this morning.

I get the UAC prompt, which says "To continue, type an Administrator password and click Yes", but it doesn't display the administrator tile, and there's no password box to type into. The Yes button is disabled.

I've just installed Windows Server AppFabric (KB970622), which installed three other hotfixes (KB980423, KB983484 and KB2300535). I guess that one of these broke it, but which one, since none of them appear to have anything to do with UAC?

I've just uninstalled Windows Server AppFabric (KB970622), but left the other three hotfixes, and the elevation prompt is working correctly. I'll try reinstalling it and see if it breaks again...

Update: yes, that broke it again. I'll uninstall it and go without.