Hi Alec, Vicente,
Vicente -- apologies for the delay in getting back to you. I can replicate this problem in OSX, Mountain Lion: I could log in once with the credentials you provided, but after logging out I can't log back in -- the page initially redirects to the dashboard, and then re-redirects to the login page. Similarly, after using the language picker to change to English, trying to choose another locale doesn't work -- the page just refreshes with English still chosen.
I'm not sure what you can do to check the cache files, but Alec may have some further suggestions. I'll be checking on a few other platforms momentarily.