can't reach dashboard on subsites - redirect loop

I can log in via the front end and the admin bar appears at the top of the screen, but when I try to jump to the dashboard on any of my sites, I get sent to the wp-login.php page and get stuck in a redirect loop.

Tried network disabling all plugins and switched the main theme, but no luck.

A bit freaked as I just launched this network and now users can't access their sites!