No preview when inserted URL

When I insert an URL in a post, the preview box shows this:

Sorry, we were unable to generate a preview for this web page, because the following oEmbed / OpenGraph tags could not be found: description, image

Which URL? Works for

Example with others forum links like PunBB or PhpBB

The replies are lacking <meta name="description" content="…"> and <meta property="og:image" content=…> so OneBox can’t preview. (So does actually, so unlike claimed previously that one can’t be previewed either.) Compare that with