You need to sign out of the child site and sign in again after fixing it.
If at any time when you sign into the child site the login form says user id instead of email then the site is broken don't login, you will have errors, if you already logged in log out of the child site.
then login to the root site and go to site settings and save it. this will change the child site back correctly to useemailforlogin
now when you go back to the child site it should say email address not user id and it is now safe to login and no errors will happen.
and it should not revert back because of the code fix if you really have the latest code.
Might be good to clarify here exactly how you grabbed the latest code. You need to pull changes in repository explorer, then close repository explorer and right click the top folder and choose TortsoiseHG > Update, this will apply the changesets to your working copy, next rebuild solution.
Hope that helps,