I could easily see antivirus software being the cause, but I just checked on the two computers that I've used to reproduce the issue and found that there is no anti virus software on either. Thanks anyhow, kbenson.
I now believe that the issue is Roku devices overheating.
Here is why. After I had used my new Roku heavily for about a day, I found that the upload failure happened. The failure was intermittent, though. Then it became more frequent. When it became so frequent that I couldn't work at all, I tried going back to my first Roku device. It worked!
Why was it working after a couple days of being powered down? Hypothesis: it had cooled down, and when that happened some system component that fails under high heat comes back to life. My guess is that the failing component is RAM.
For developers the takeaway is to own two devices and switch between them when uploads on one start failing. Turn the one that's failing off in the meantime, so that it can cool down and be ready to use again when you need it.
What's the ambient temperature? Is the Roku on some other device that generates a lot of heat? I haven't seem this behavior, and I have no A/C, but I do most my development at dusk and after. That said, it's been somewhat warm a few times (~80 degrees), and I haven't seen this problem. I haven't seen any overheating issues on my other unit, which is dedicated to normal use, and I've used in MUCH warmer temperatures (> 100.degrees).
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