Login Loop on Multisite Sites (Not main site)

Hey

I have a multisite setup running WordPress 3.5.1. It's a subfolder setup and I currently only have 1 additional site setup. I have the WPMU DEV Domain Mapping plugin installed and the subfolder site (maindomain.com/subsite) is now running happily from it's own domain (subsite.com).

Unfortunately as soon as I activated the domain mapping for the sub site I can no longer login to it's admin panel as I am stuck in a login loop. When I visit the login page (subsite.com/wp-admin/) I immediately get redirected to the following URL:

subsite.com/wp-login.php?redirect_to=http%3A%2F%2Fsubsite.com%2Fwp-admin%2F&reauth=1

When I enter the username and password the page refreshes back to the login page, provides no error message and clears the text boxes. I can't login at all.

I have tried the usual stuff like deleting browsing data/cache/cookies/etc and I have also tried using different browsers and computers (Windows & Mac).

I can login to the main site (mainsite.com/wp-admin/) absolutely fine and I can access all the sites to make updates. The problem only happens once I enable the plugin.

Does anyone have any advice?

Thanks,
James