1) When the user is logging off, are they doing it with the SCLogin module or somewhere else?
** Can you enable the standard Joomla login module (mod_login) and test with that to see if the same issue occurs?
answer: This is not a login module. Login menu joomla:
dobrzeogloszone.pl/formularz-logowania
2) Does this happen when the user authenticates with a social network only, or when they login with standard Joomla credentials as well?
The screenshot is a little grainy. Are there any error messages in there or anything else we can use to understand what the problem is?
answer: This is what happens to any user logs on, not only through social networks.
They do not show the specific errors, but it looks like to be duplicated page on which the user is transferred when you log off.
It would be best to register and see live how insight