You can add the static form code to your main site's code (your domain in this case is example.com and your subdomain is sub.example.com):
<form name="loginform" id="loginform" action="https://sub.example.com/wp-login.php" method="post">
<label for="user_login">Username or Email Address</label>
So, we figured out what it was.
In the options table (wp_options for some, we chose to do away with the wp prefix), there's a user_roles (or wp_user_roles value). One of the plugins we used (Yoast SEO in this case) had an update that zapped the value of that option, removing the Administrator role from that list. As a result, no admin could log into the ...
The correct answer is to use SSL.