The AJAX failed or the nonce verification failed. If you encounter this error repeatedly, it is possible that access is being blocked by a security-related plugin. Please try changing the values of "Επιλέξτε τη διεύθυνση URL για το AJAX στη δημόσια σελίδα" and "Select a function to validate the value of a nonce with AJAX on the public page" in Booking Package > General Settings.