Error page when signing-up with GitHub

Signing up with GitHub leads to this error page, although the notification email is sent and the user is subsequently able to log in.

Start by reproducing the GitHub signup flow and tracing where the error page is selected after the notification email is sent. The issue is done when a successful GitHub signup reaches the normal post-signup page without the error, while email notification and subsequent login still work.

Written by the indexing model from the issue text.