Appointments page is "disabled". You cannot select any days.

I have created a service and a service provider and added them together. When I insert an shortcode into an page, I see the calender, but it is disabled and a red sign appears. It also seems like the css is not working. This is a multisite, but this is the only plugin installed on this site. Am I doing something wrong? Or is this a conflict with something else I cannot see? I have the newest version.

Thanks for your help.