Comment 9 for bug 1130945

You might try to change your username on launchpad and Ubuntu Single Signon (https://login.ubuntu.com/) to the same. Does it work permanently for you then? Because this error occured for me after changing my username on launchpad while Ubunut Single Signon still had the old username. I've changed them to be the same again now and manually edited the "/etc/apt/auth.conf" with "sudo nano /etc/apt/auth.conf" (you must do it with root privileges!). At least after installing a free application on the Ubuntu Software Center, the username in the auth.conf wasn't changed back. So this might be worth a try.