Date/hour availability doesn't work

Hi there,

I’m using the Plugin on a customer page and the dates aren’t available anylonger. They were green, now they’re grey and I cannot choose a date or an hour to book.

As far as I know, no changes were made in the backend. Using WordPress 4.2.7

You can see it here.

Thanks for your help.

https://ww.wp.xz.cn/plugins/booking-system/