Comment 1 for bug 1212948

This is not a bug. You cannot login via Launchpad the way you could prior to the forum upgrade in February. The way to log into Ubuntuforums is via Ubuntu One, which is "the single account you use to log in to all services and sites related to Ubuntu."

If you don't have an Ubuntu One SSO account, go to https://login.ubuntu.com/ and create one, as advised early on in the sticky you link to. Please be sure to set your preferred email in Ubuntu One to match your forum account, otherwise the system will create a new forum account.