REQUEST FOR APPOINTMENT Error!

I tried to request an appointment from my embedded calendar. It processes the request and sends it to my email account.

In the email, when I click "Please click here to accept the appointment"

It takes me to soho page and I get:

    REQUEST FOR APPOINTMENT
    Error!
    Sorry we are unable to process your request. Please try again later.

Appointment is not added to calendar

What is the problem?