Fatal error on Export Date Range add-on

Trying to export the appointments using a data range, it gives a 500 error. This is what was saved in the debug.log:

PHP Notice: ob_end_clean(): failed to delete buffer. No buffer to delete in /wp-content/plugins/appointments/includes/class-app-ajax.php on line 1020

PHP Fatal error: Call to a member function format() on boolean in wp-content/plugins/appointments/includes/pro/includes/addons/app-admin-export_date_range.php on line 35