Appointments+: Subscribers can't book appointments

Subscribers can't seem to book appointments, administrators can.

When a subscriber clicks on an available date, it shows the bookable time slots, and when they click one of those, they get an 'infinite spinner'. Nothing else happens.

My client hasn't been able to receive bookings because of this. Please help urgently. I can't give WPMU support access to the site listed above, because I just purchased the plugin, not a membership...

Thank you.