I've tried the sharing on your site and can confirm the behavior, but unfortunately the sharing debugger from Facebook (
https://developers.facebook.com/tools/debug/sharing/) isn't giving the same results -- it's indicating that the page is fetched successfully. That makes debugging this very difficult.
The only thing I really notice is that your board URL appears to be http but you're forcing https. Normally, this would 301 though so I don't think that's necessarily the case, but worth checking first I think.