I am having a similar problem.
I recently upgraded my blog to version 2.7. and really like the new back end, but the WordPress Stats will not allow me to login from my dashboard now.
I can login into wordpress.com to see my statistics, but from my blog on my server, I try to login in on my dashboard and I still get a login page.
I also tried different templates and still the same problem.
I reentered my API, I checked my username & password. I am using the latest version of WordPress Stats 1.3.5
I can see that data is being collected on WordPress.com, but cannot access the stats from within my blog on my server.
I originally posted this problem on wordpress.com and was asked to post this message here.
I am at a loss.