Support

Most likely you are hitting a server firewall. The CoreSums come from https://getpanopticon.com/checksums/index.html which is served over CloudFlare CDN. If you can whitelist a domain name, that would be getpanopticon.com and www.getpanopticon.com. If you need to whitelist an IP you need to resolve the domain name from your server since this being behind a CDN you see a different IP depending on where you are.

You can of course download the very same PHP software generating the CoreSums as we use and host it yourself. On the page I linked there's the “Reproducible, auditable content” which tells you where to find it: https://github.com/akeeba/coresums It only uses very basic PHP to generate static files you can serve any way you like – and there's an option in Admin Tools to point your site to your own copy of CoreSums. We do that on purpose. We do NOT want people to blindly trust us in matters of security. We want all security-relevant services we provide to be fully auditable by anyone, and possible to host yourself so you can cut us out of your security risk analysis.

I hope this helps!