Seems that they standard point the cdn to the latest version and that broke with the 5.0.0 release from yesterday. Here a fix. Edit all the urls from the fullcalendar to include a version number:
<script src="https://unpkg.com/@fullcalendar/core/main.min.js"></script>
<script src="https://unpkg.com/@fullcalendar/core/locales-all.min.js"></script>
<script src="https://unpkg.com/@fullcalendar/interaction/main.min.js"></script>
<script src="https://unpkg.com/@fullcalendar/daygrid/main.min.js"></script>
change them to:
<script src="https://unpkg.com/@fullcalendar/core@4.4.0/main.min.js"></script>
<script src="https://unpkg.com/@fullcalendar/core@4.4.0/locales-all.min.js"></script>
<script src="https://unpkg.com/@fullcalendar/interaction@4.4.0/main.min.js"></script>
<script src="https://unpkg.com/@fullcalendar/daygrid@4.4.0/main.min.js"></script>
You can have more includes depending on the modules being used.