I ended up at this page: https://badges.mozilla.org/en-US/browserid/browserid/verify/ – with a 403 message "CSRF verification failed. Request aborted."
My actions before: go to a badge claim page, logout, get redirected to home page, log back in (different email). In the end I seemed to have been logged in.
Successful attempt to reproduce: on home page log in, log out, log in again.