I have the same problem and I have been investigating it.
This is how the hide feature works.
1. Your secret custom login/admin url just redirects to the normal wp-login.php with a secret string appended.
2. If anyone tries to connect to wp-login.php without the secret string, wp-login.php pretends not to exist and redirects to a 404 page.
I discovered that this problem occours because the secret string is not appended to the url in the action attribute of the form html tag. The result is that the form is posted to the url without the secret string and the result is a redirection to a 404 page.
This happens only when trying to connect to mapped subdomains. Login to main network domain works perfectly.
Any suggestions on where to look for solutions are welcome.