What does this mean?
Hi, can you share what you were uploading? You can send it to support@zztakeoff.com and we can help you get this resolved asap.
One of my estimators got this error too when uploading files also. What causes this? 
Hi, a 502 error is an intermittent error that is not directly caused by one issue. It could be a timeout, or any other network gremlin that was inside Amazon Web Services. How often do you see this issue?