I have no definitive answers but think about other possibilities everytime I see a new post. Here are some thoughts... Is this specific to the user who is logged in? What happens if they use a different device? Different user? I often wonder if it is a locked file on a particular device or if on all devices. What about "incognito mode or guest mode? I know back when I was trouble shooting I uninstalled Chrome, deleted all reference in every folder and temp I could find..tried different browsers, different videos, but I don't recall if I completely reset the network settings. I think I would remove/forget all wifi.
Clear with netsh int ip reset at admin cmd prompt, ipconfig /flush dns too. reset IE, reset Edge, reset Chrome, sign user out first. Start fresh.
I reset the device that was doing it to me back then...have nothing to test my theories...if any of this works ..please let me know.
Good luck!