WordPress Super Admin cannot create User in Admin Area

I cannot add a new user account from the admin area as super admin on the core (network) site or the main blog site of wordpress.

When I click on Add user I enter the user name and email address but after I click on Add the screen flashes but returns the the Add new user screen with both fields blank. When I check the All Users area the new users are not there. I’m running the latest install of both WordPress and s2Member Pro.

Any ideas where I can start?