I’m getting the error as well. from https://www.instagram.com/developer/sandbox/
“Any app created before Nov 17, 2015 will continue to function until June 1, 2016. On that date, the app will automatically be moved to Sandbox Mode if it wasn’t approved through the review process. The previous version of our documentation is still available here.”
Looks like the plugin developers might need to reapprove the plugin through IG.
SOLVED: (sort of).
I switched to another plugin, Instagram Feed. https://wordpress.org/plugins/instagram-feed/