My login is looping on itself and redirecting back to login screen

On one of my multisite installs after upgrading to wordpress 4.0, I have the following issue.

When I try to login as admin or as a user on any of the subsites, the login page refreshes and redirects to itself.

I've tried the following:

Renamed plugin folder - no effect
Deleted htaccess file - no effect
uploaded new wp-admin and wp-include folders- no effect
added url to wp-config - no effect

I'm running out of things to try, is their anything you can suggest?

Thanks
Ian