When adding a user to my whitelist I receive a "That player does not exist" error. Here is a similar ticket, WEB-4457, which explains how the whitelisting curl request works.

I am able to run the curl request on my residential network so it appears the server is not down. However, I asked my host to run the curl request on their node and it appears that Mojang's CDN, CloudFront, is blocking the connection. I asked my host try it on a node in a different datacenter location, thinking I could just transfer my server, and they encountered the same issue. It would appear for that CloudFront is blocking datacenters from making the curl request necessary for whitelisting. This is problematic because many Minecraft servers are run in datacenters. I have attached a screenshot of the CloudFront error.