Feature image not showing

Hi.
Post Grid is used on 2 websites I maintain.
Until recently it worked fine on both sites.
But now on 1 site the featured image is not shown anymore ((Media Source Feature Image). See link to the page.
When I switch Media Source to First Image from content that image is shown.
* I have compared all the settings on both sites
* I have put the problem site in Troubleshoot mode with only Post Grid activated: no effect.
The Post Grid is on both sites on version 2.3.0.
Wordpress is on version 5.2.2.
What could it be that causes this?

Thanks, Geert

The page I need help with: [log in to see the link]