I am running EE 2.1.3 build 20101220. I just upgraded from a Freelancer license to a Commercial license to use the member module functionality. When I visit <site_url>/member/login I am presented the login page.
When a registered user (Members Member Group) tries to login, the user is redirected to a blank page. The url is for the home page of the site.
The same thing is happening when I try to use the Member Registration from the front end. I can see the Member Registration form, fill it out completely, and when I click Submit, I am again redirected to a blank page. The url is for the home page of the site.
I can login with my SuperAdmin to the EE CP just fine.