Could it be an problem with the hosting?
Because I have the hosting at the hostinger.it,
and when I have tried to use an other hosting: demojoomla.com,
there the Facebook plugin of the SLogin with the same settings of the FB app (ho changet only URL Callback) works very well.
The Joomla!® name is used under a limited license from Open Source Matters in the United States and other countries. joomline.net is not affiliated with or endorsed by Open Source Matters or the Joomla! Project.