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.
Typically they close the ability to access social networks and, most importantly, they close the ability to access the server for social networks.
The social network, in other words, turns to Slogin...
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