If you change the masterpassword, and then still get the same message after that, then this means it is not from some old hacked data store, but it is someone able to glean the latest masterpassword from your account live.

I saw this on twitter, but cant reply as I am in twit jail: "Exactly the same thing happened to me last night. They tried again literally minutes after I changed the password to something not used on any other form."