Form Send Button Refresh Issue

After completing the form in front end and clicking the send (book) button the webside does not show the after-send notification message and does not refersh the button which is stuck in reloadnig.

Have a look here at the bottom at the page:

https://warsawrunningtours.pl/blog-warsaw-running-tours/warsaw-running-tours-blog/84-onepage-blog/94-warsaw-second-best-in-the-world-for-clubbing-running-warsaw

The form works fine and sends the email notoficatin. But the send bottom is the problem, beacuse it does not refresh.