I realized in the past month that often when I have had trouble trying to resume an upload which had stalled, my problem is that my cookie has expired + I am no longer signed into my account (although the tab that the upload page is open in still shows my username in the upper right, since that part of the page hasn't been updated/refreshed since I began the upload). The only way to discover this is to open a new Tech Preview tab (the homepage works fine) and see whether one is still signed in.
It is unclear to me whether this might be due to me signing into my account in another browser on the same machine or signing into the same account on a different computer in the house.
Perhaps sometimes I fail to check the 'Keep me signed in' checkbox on the Microsoft Account login page?
I'd like to get to the bottom of why this is happening (it's unusual to get signed out of your account in the middle of an upload) and do my best to avoid triggering this behavior anymore.