Test times out on private instance but not on webpagetest.org

Here’s a test run from my private instance (published to webpagetest.org) that timed out while the main resource was still loading:


Here’s the exact same test run on webpagetest.org itself, and there’s no time out:


I’m using WebPagetest 2.15. The test agent is running on an EC2 m1.medium using ami-c262d9ab.

Any idea why this is timing out on my private instance?