This is about the trac ticket you posted. Can’t register again with same email address after deleting user. I’m just hunting you down since you might not read the comments on the ticket.
That is a side effect of mu storing the users in wp_signups table and not getting rid of them from there when deleting a user. Don’t ask me why, it just does. If you want to re-use that email address you have to kill the appropriate rec in wp_signups.