BuddyPress Login Sidebar Weird Behavior Desperate

Dear people,
Working on two very similar sites (both multi-site and buddypress).
http://redebrasilis.net
http://razoncartografica.net
Well, if you just click on the field “username” you’ll see what’s happening: razoncartografica.net works well, I mean, it allows you to enter text, as expected…
But, on the other hand, redebrasilis.net loginbar keeps redirecting you to the sing-up page from the very moment you put your cursor on…
I know other ways (links) to get people to the login form but… Whay does this happen????? How can I fix it?????
Thanks!