My guess is you are having your crawl rejected (and this is happening here on meta too).
But I have a solution, but it’s not 100% free:
I just tested this on two of my sites, one equipped with the plugin, and one without, and it works on the one equipped.
One of the issues here is that Discourse/Onebox could be a heck of a lot more transparent about the way it handles a failure. I guess that’s a stretch feature, but it would be nice if there was more feedback to the user pasting the link (or even just the admins).
I checked my /logs on the site which failed and do not see any extra information wrt to the failed preview …