As we can see, your Facebook APP is still in development mode and that is why Facebook login doesn’t work https://gyazo.com/10961c3d37d06d7c569a25a4675aa159
Fix this issue first and contact us so we can continue.
It seems that the URL to your account is wrong on this screenshot. Please, correct it first. As we can see, your current account URL page https://www.snapmaster.de/products-page/your-account/