@samnine
Your homepage outputs this image and Facebook returns the following error:
{image} could not be downloaded because it exceeded the maximum allowed sized of 8Mb or your server was too slow to respond.
Unfortunately, this either means the image is too large (not likely) or something in your setup is causing your server to respond too slow. The fastest way to rule out conflicts is to deactivate all non-Yoast plugins and switch to a standard theme like Twenty Sixteen. If this resolves the issue, you have a conflict and should activate one plugin at a time until you locate the conflict. Finally, after all plugins are active, switch to your theme. This will narrow down the conflict to a specific plugin or theme. If you are unfamiliar with checking for conflicts, we wrote a small guide that will walk you through the steps. You can find it here: How To Check For Plugin Conflicts
If you do not find an internal conflict, this means your server is responding too slow. Please contact your webhost for assistance with server issues.
The course example page outputs this full size image which, for an unknown reason, does not work. The smaller version, however, loads correctly. Have you intentionally removed or restricted access to the full size version of this image? If so, please manually set the Facebook image to the correct image URL. If you haven’t removed or restricted access, please run a conflict check to determine what in your setup has caused the restriction.