I finally found some time this evening to start digging into an issue with Healthcheck notifications in the Speedtest-Tracker project.
From what I understand when reviewing the code is that the URL of the Healthchecks instance and the UUID of the check isn't being passed in properly, nor is the "ping" URL being formed correctly which is causing 500 Internal Server Errors and 404 Not Found errors. After manually setting the URL and UUID I was able to get it working.
Next up would be figuring out how to properly fix this so that variables can be used instead of manually setting things.