Embed URL not working

I often embed URLs to pages or posts (by pasting a URL on a separate line which creates an Embed block). These URLs all point to posts within the same website. This has always worked well. Recently that has stopped working though: I can see a <figure> element on the page but it has no content.

In the block editor, the embed is showing correctly. In the default Twenty Twenty-Three theme it works fine too.

I’ve tried disabling all plugins but that did not solve the issue.

The page I need help with: [log in to see the link]