Hello.
1) This issues is possible, when you try to load the page, where DNS is different from that you are configured at the WordPress General settings.
For example.
If you are request the site on the following URL http://www.server.com/my-booking-page/
but main part of your scripts are loading from this URL (without "www") : http://server.com
so then you will have that type of issue. You are also can see the "0 error" in the browser.
The Ajax request (its the request when we are making new booking) is possible to execute only to the same site, which is loaded in the browser URL, so thats why we are get this error.
Probably you are need to set the correct URL at the WordPress Settings General at the WordPress Address and Site Address.
2) Please check other similar issues at the FAQ here http://wpbookingcalendar.com/faq/ at the troubleshooting section.