I've had this happen on a cell phone modem! My (large) image uploads to an online flow-chart saas started failing. Open up the chrome dev tools, turns out they all failed at exactly 20.0 seconds -- obviously a timeout issue. I had a maddening support experience in which the support tech kept insisting that he couldn't reproduce the problem from his (high bandwidth) end. I asked him to please just ask an engineer to raise the 20 second timeout, but he stone-walled me. So frustrating.