Activating WordPress SEO Leads to WordPress Login Screen

When I try to activate WordPress SEO I cannot access the admin area of my site as the login screen keeps coming up, and trying to login does nothing as it just stays at the login screen.

Iv deleted the add-in via FTP to regain access to the admin area but would like to get WordPress SEO working – Any ideas?

I am trying to run the latest version (1.3.4.4) of WordPress SEO and version 3.5.1 of WordPress.

http://wordpress.org/extend/plugins/wordpress-seo/