Just like Marvin, https://easy-appointments.net/support/undefined-error-when-booking-an-appointment/
 
I experience the exact same problem.
1 out of 10  times it randomly works, but in all other cases it gives the undefined popup. ( website says: undefined )
 
It does block the time slot, but no data gets registered, and you won`t get Detoured to the thank-you page. which you normally would.