Problem With Featured Image

When I edit a page where the featured image was set quite some time ago (and is being used properly) and I want to change the featured image, the menu area indicates a featured image is not set. When I try and set it, the media library fails to load any images, just get the busy circular ring.

I did recently add the “Featured Pages” paid add-on and updates Visual Composer.

Not all pages have this problem, but most of them do.
my website is https://pembrokeinstruments.com