The modal popup is displayed by the default AJAX error handler (*AjaxErrorJavaScript* property):
AjaxErrorJavaScript="if (DayPilot.Modal) { new DayPilot.Modal().showHtml(args.request.responseText); } else { alert('AJAX callback error (500)'); }"
You can replace it with an empty string to disable the modal popup:
AjaxErrorJavaScript=""