Domain mapping is rewriting the site url on wp-login.php to use http instead of https.
I have granted support access. If you look at blog 38, I have updated the site url to use https but the subsites wp-login.php wont work because the form action is using http.
If you disable domain mapping then the login form action points at https and it works fine.
Domain mapping is re-writing https to http for some reason. I need to enable domain mapping without breaking all my subsites login.