I do not ‘play’ too much with the multisite environment. Maybe someone can help enlighten us. But these are what I know:
– Normally, WordPress users use example.com/wp-login.php to login.
– On multisite environment, users also use example.com/wp-login.php to login into the main site.
– On multisite environment, if you want to login to your subsites you also use the same login: example.com/wp-login.php.
– So what is the difference to make the login to go to main or subsite? The Username you provide when login. If the user is a Super Admin then after login, he/she can access all the main and subsites. But if the user is only a ‘ordinary’ admin, he/she can access only the subsite that is associated with him/her.
So, if you now can login to your main site, it means the problem solve. I hope you understand what I mean.
About the ‘wp-admin’ you mentioned, don’t use it because it is not a normal login url.