error sending form

If you can send it but he can’t, maybe it’s a firewall issue? Have him try from a home computer if he’s experiencing this at work.

Now I get more information about this:
I have pasted this form in plugin with name All-in-One Event Calendar by Timely. If I have Your form in post it works and when I have Your form in this plugin it sends me this error.
And it soes not matter who wil apply this form.
This form You can find here: https://irlat.com/ai1ec_event/late-breakfast-networking-e-health-advantages-and-health-tourism-today-and-tomorrow-at-it-demo-centre/?instance_id=41

Are you running the latest version?

It occurs when you redirect from one page to other, even when on same domain. To suppress, open /includes/email.php and comment out:
if ( $wp_get_referer )
wp_die( __( ‘Security check: referal does not match this site.’ , ‘visual-form-builder-pro’), ”, array( ‘back_link’ => true ) );