JS Error: Backbone is not defined

Hello!

After updating a page, the contact form is no longer loaded.
The error ” Uncaught ReferenceError: Backbone is not defined” is displayed in the Chrome console. File: front-end.js.

In WP backend I can edit the form normally.

I cleared the Cache (I use a Plugin) and tried it with a private window.
I’m using the Theme Enfold.

If you need more informations, let me know.

The page I need help with: [log in to see the link]