I was doing debugging on a different profile (called "Debugging") on Iceweasel (Firefox on Debian Linux) and other one called "Entertainment" (and Google Chrome too), and for my surprise I can't login on none of these, but I still can on the usual profile (called "Development").
I tried activating the admin user, with no success. It's the same with develop and production environments, and there's no error message in the Symfony or Apache logs.
Symfony 2.3.9 SonataUserBundle 2.2.3 FOSUserBundle 1.3.3