Though I can’t think of a reason why, it looks like Facebook is at fault. It’s been known to happen. Maybe the scraper recognised you’re on a different version of WordPress and needed to be forced to re-index. But I’m speculating there.
Anyway, simply hitting the ‘Fetch new scrape information’ button on the FB debugger page forces FB to get the new info from your site, and now it’s perfectly fine: https://goo.gl/R4XuHM
As mentioned the output of my plug-in has not changed, and FB reports no open graph errors (even the one you mentioned).
Bear in mind that any existing posts you have on Facebook that are showing the wrong thumbnails will continue to show them as such, and you’ll need to either just live with that or delete the posts and add them again (not that FB has the latest scrape info it should be fine for all future posts).
Have fun – I’ll consider this issue solved.
Mike.