I have confirmed this to be a general problem introduced by KB4056892 (and we now know that KB4056898 for Windows 8.1 has the same security change)! I started with a clean Windows 10 system with only ASCOM Platform 6.3. I tested POTH connecting to the ASCOM Telescope Simulator for .NET and the old ASCOM Telescope Simulator (VB6 Based). No problems. Then I manually installed KB4056892. After that here are the results:
Rowland Archer nailed it in the ACP Forum discussion thread by posting the "known issue"
This is it. See the next post for how to solve it.