I had two user accounts, one admin (let's call this Admin1) and one with restricted privileges. I then created a second admin account (Admin 2) which did not have a password and deleted the original admin account along with the other restricted account.
Now when I try to update OS X, it asks me for the password of my Admin1 account which did have a password. I enter it but it's rejected. But why would it be asking for one in the first place?