Error/Status code -1

We got a report from a user not being able to see part of our site, after further investigation we were able to replicate on WebPageTest. One of the items I can’t figure out is a Error/Status code -1. Does anyone have any idea how to debug that?

Here is the test as reference:

-1 is a default error code for when a request is sent but there was no response (or the response wasn’t detected by WPT).