I did hear back from them via email. See below:
Thank you for interest to the Booking Calendar.
You can check for the most common issues on the FAQ page https://wpbookingcalendar.com/faq/ at the troubleshooting section.
You can also 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.
Here is a screenshot of what I see when I do Ctrl+Shift+J….of course I have no idea how to fix it!