Appointments+ Date Picker jQuery console error

Hello,

When I click on the appointments picker (http://puu.sh/kBF1M/2b75a314ae.jpg) , it used to open up a window to the right where it'd show the times. But now it doesn't anymore and it throws this error in the console;
Uncaught TypeError: Cannot read property 'createDocumentFragment' of null

It literally worked, and then an hour later the client replied that it doesn't work when they tested it (and it doesn't).

About a day later, it just worked again, without anything being changed.
Then; a day or two later, it broke again and it's still broken right now.

I've posted this issue before, but I'm not getting a reply anymore so I'm hoping making a new topic might get a reply. (https://premium.wpmudev.org/forums/topic/appointments-datepicker-jquery-doesnt-throws-error)

Thanks,
Wouter