Error when I confirm the appointment

Hi, when I get an appointment and I confirm it from the booking control panel, two things happen to me:

1) I click on accept and it takes me to a page with the report of the appointment in question and below it says “critical error of wordpres etc.”
2) the confirmation email does not arrive to the customer, while if I place the canceled order, the customer receives the email.

How can I solve? Thank you.