Sometimes when posting a link, something goes wrong and the preview doesn’t come up properly.
I hit this issue yesterday when posting a link from my personal blog. At the FB probe went looking, it seemed to hit a database error, so that’s what it said. Not very helpful.
I tried the site itself, and it was up. Retrying from FB didn’t fix it – it was now in the cache. Thankfully there’s a way of refreshing it:
Go to https://developers.facebook.com/tools/debug/
Enter the URL and it’ll show you the current cached preview image, and any errors.
Click the Scrape Again button to refresh it.