Why do I get a 403 or 404 error instead of being logged in?

I've set up the drupal openid and launchpad plugins on my site and everything seems to work ok, but when I try to login I get a 403 error when I return and I can't log in.

This can also appear as a 404 error in the browser but the server logs reveal a 403 error and the server then can't find the proper error page to display.