I've encountered a few problems in this space.

1. Comcast/Xfnity/Firewall deciding they don't like the proseries.com domain.  Short Term Fixes have included turning off Protected Browsing or Whitelisting proseries.com.  Long term we're working with our IT folks  to address what's getting us flagged (forever, our server side certificate has had turbotax.com as the primary and proseries.com as an alt.. evidently these days some providers don't like that arrangement)

2. Sometimes router DNS caches get corrupted. Usually a hard reset (unplug from power, wait a minute, plug back in) clears the cache.

3. An Anti-Virus / Anti-Malware program like AVG suddenly decides to quarantine  ProTax20.exe and/or CefBrowser.Subprocess.exe.  I'm not familiar with any particular reason for that. 

I'm not familiar with a specific reason why updating Windows 10 would have helped, but am glad that it did.