Multisite – Suddenly cannot log into any subsite

I’ve come upon a bit of an issue today, when I was trying to log into a subpage in a multisite – when logging in, it would just revert back to the login page with 0 feedback.

I’ve tried:
– reuploading all core files
– reverting to default htaccess
– disabling all plugins
– disabling all themes (using twentyten)

When i put in a wrong password, it still says wrong password. But on a successful login, it just redirects back to login.

I can login to the main page just fine.

Please help!