The problem now might be caused by the domain or hosting. These are the options:
– Your domain only allows a sender from the same domain. In this case, you should change in the widget settings “From email: firstname.lastname@example.org” for another email with the domain, such us “email@example.com”. Once changed, the form should work.
– If the above does not work, that is probably because your hosting is not PHP compatible. You can try and contact your hosting and see what they can do.
The widget instructions include the note below. There is a link where you can read more about this problem.