I installed the free plugin and created a post grid, used the shortcode on page and it appeared as I expected.
When I purchased the premium plugin, I removed the free plugin, installed the premium plugin. The original post grid I created under the free plugin shows up in the post grid list. But the page on which it is displayed shows 503 error. I created a second post grid with the premium plugin and when I place this on page I get a 503 error on that page. When I remove the premium plugin and reinstall the free plugin, it works fine.
I then created a new page and posted the shortcode. When I publish and try to view that page I get a 500 error, but only for that page.
So the premium plug does not work on my site, but the free plugin does.