After March 2018 you have to put the exact return URL into the Valid OAuth Redirect URIs input. Previously, with strict mode disabled, you could just put your domain name in and that would be enough. Now it has to be the exact URL that your return to once you’re authorized Facebook. You can find this link just above Facebook settings in Yith Woocommerce Social Login plugin.
Valid OAuth Redirect URI: https://yourdomain/wp-content/plugins/yith-woocommerce-social-login-premium/includes/hybridauth/facebook.php
