This usually happens if you have entered the wrong details when you created the App in Facebook. Or have you changed a URL's of an existing App?
Can you please recheck the settings of your APP in this page?
https://developers.facebook.com/apps
Select the correct App and click in the edit button;
Check the URLs & paths are correctly entered and are pointing to the site where you have installed Ultimate Facebook plugin.

This usually happens if you have entered the wrong details when you created the App in Facebook. Or have you changed a URL's of an existing App?
Can you please recheck the settings of your APP in this page?
https://developers.facebook.com/apps
1. Select the correct App and click in the edit button;
2. Check the URLs & paths are correctly entered and are pointing to the site where you have installed Ultimate Facebook plugin.