This bug appears identical to that described in
I am unable to update or install software in the AppCenter
however none of the fixes suggested there (reinstalling packageit, reinstalling appcenter and all the dependencies needed to build it) worked.
Problem started when I accidentally hit "Authenticate" prior to typing my password during a routine upgrade in appcenter, after which it wouldn't let me type in my password any more, so I killed it and tried again, with the above results.
HELP!
