Can’t embed MS Outlook web calendar in WordPress

Back in 2017 we embedded our corporate calendar in our WP site. It is a normal Outlook calendar on Microsoft 365. It all worked fine for years. The calendar on our site was always updated from the 365 one. Now it does not work. Meanwhile we are setting up a new site, and I can’t embed the Outlook calendar at all. WP just reports that the content can’t be embedded. There is a list of sites that can be embedded, and no Microsoft site is on it. What has happened? Microsoft support has no clue as to what is going on. Can I only do this by installing a plugin? I didn’t need one before.