Symptoms
One situation I ran into when I migrated 1keydata.com to HTTPS with HTTP2 enabled was that I had an issue with the page load speed testing tools WebPageTest and GTMetrix. On both tools, https://www.1keydata.com/ failed to load, yet when I visit the site using a browser, the website loaded up fine. This is the first time I have seen a difference in behavior between these types of testing tools and a browser. Given the highly unusual nature of this difference, I decided to look into the issue further. Below is what the test result page on WebPageTest looked like:
