Hello, This is the description of my problem:
1) click on www.mysite.com/index.php/component/users...munity&view=register .
2) click on the button "Iniciar sesión Facebook".
3) click on the button "aceptar"
4) I get a blanck popup screen

Hi Juan,

something wrong with your site. your site cannot access mysite.com/index.php by AJAX request. seem there is some miss configuration at your server. it might be htaccess or some blocker at your server.

first try to delete any htaccess file first under joomla root. dont forget to backup that file first.

Regards