This issue is relative to the configuration of the DNS in your website.
Please try to reconfigure it or contact your administrator for fixing this issue.
Description of reason of the issue.
1) This error "0", can be because in your server configuration somewhere (probably in the WordPress > Settings > General page for the options "WordPress Address (URL)" or "Site Address (URL)") configured different Domain that you try to submit the booking.
For example, in Ajax URL its can be set as "https://www.yourserver.com/wp-admin/admin-ajax.php"
and you try to submit the booking from http://www.yourserver.com/bookings/ Or backward.
You can check it is by viewing the HTML code of your loaded page, in the section:
<!-- Booking Calendar Scripts -->
Because booking is sending from HTTP to HTTPS, we are having such type of error.
Please recheck your server configuration at the WordPress > Settings > General page as in above description, or in some other place, where you are set using HTTPS instead of HTTP protocol.
You need to use the same protocol in both ways. If you will have difficulty with searching where exactly you are having this issue, please contact your server administrator who make that configuration.
The same issue can be if you configured sever address with "www." But trying to submit booking from URL without "www." prefix in URL. Or backward.
2) Also please try to deactivate one by one all your active plugins at the WordPress > Plugins menu. Then retest it again.
If its not help, please deactivate your active theme and active the default WordPress theme. And then retest it again. May be there exist some conflict with some active plugin or actual theme and just need to find the reason of that issue.
If the previous solutions will not help, please recheck your error.log file from your server configuration (please contact your server administrator or support of the hosting company, if you will have troubles with searching of this file) to be sure in a reason of that issue.