WebPageTest Error With HTTPS / HTTP2 Enabled Site

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, http://www.navlit.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:

WebPageTest Error Result Page

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