Failure parsing JSONin fullcalendar

Question

I am having problems with fullcalendar, some time ago this problem was not happening and suddenly it started to send me this error.

enter image description here

The code I am using for fullcalendar is as follows

document.addEventListener('DOMContentLoaded', function () {
var calendarEl = document.getElementById('calendar');
// var newDate = new Date(2021, 9, 13);

var calendar = new FullCalendar.Calendar(calendarEl, {
    initialView: 'timeGridWeek',
    locale: 'es',
    headerToolbar: {
        right: "timeGridWeek,timeGridDay,listWeek",
        center: "title",
        left: "prev,next"
    },


    // No muestro los fines de semana
    weekends: false,
    hiddenDays: [0, 6],
    // allDaySlot: false,

    // slotMinTime: '11:00',
    // slotMaxTime: '21:00',

    events: 'http://adm.local.com/ventanas/MostrarFeriados',

    // EventSources son los eventos que se van a mostrar en el calendario
    eventSources: [
        {
            url: 'http://adm.local.com/ventanas/MostrarEventos',
            color: '#00ff00',
            textColor: '#ffffff'
        }
    ],
    
});
calendar.render();

});

I have checked the PHP code I use and it returns the json correctly. Fullcalendar is not rendering it in the view.