Thanks for the credentials.
To give you some progress, this is related to the language switcher being enabled on your site. I found it by looking at the network URL activity in the browser developer tools. When I disable your language switcher plugin and go to your main page
www.mammacult.com/ without /it then I am able to create a FB user successfully.
This is one issue that we've had from time to time on our user's sites but we have not been able to successfully reproduce the behavior on our test sites with language switching enabled. If possible, would you be willing to provide us FTP credentials so that we can debug this further?
-Melissa