Order cannot be sent

Hi, my ordering system does not work under any circumstances.
I add items to the cart, fill in the contact details, but when I click “Submit” nothing happens and the form fields still contain the filled in details.

I have replicated this many times in different browsers, always clearing the browser cache before trying. Everything is updated to the latest version – core, theme and plugins.
I also tried in the meantime to downgrade the WordPress core and downgrade the Five Star Menu plugin (up to version 2.4.11 and then even 2.4.2). All without success.
This is a website that was already running, after a while the client wanted to terminate the order and now it has resumed again. So I restored it from a backup. However, it didn’t work either in the original state or after the update.

Now everything is updated to the latest version and all plugins are deactivated except Five Star Menu.

Thanks for any advice,
Martin

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