Cannot login to WPMU plugin within install

Hi there - I've installed the plugin on the listed domain, but if I enter my user/pass into the WPMU login page, it just refreshes the page without any error message - not happened before on any other site. I've tried removing and readding it - but the same issue occurs. It was previously connected to another account (james@code23.com) - but i removed that myself already.

Any ideas?

Cheers
James