Suddenly unable to share Event listing URLs are Page Posts in Facebook

Everything was fine before the Bank Holiday, but when we came back, it'd stopped working. If I paste an Event Listing's URL into a Page Post, it'll give me "Fetching Preview" forever, and the Post/Schedule button is greyed out.

This happens on my PC in IE, Chrome, and MS Edge. It also happens if I log in as Admin on a different FB Page. However, it doesn't seem to happen if I use Safari on my iPhone.