After an unspeakable number of hours trying to find a solution, it was these two changes (both from this source: https://answers.microsoft.com/en-us/windows/forum/all/screen-mirroring-not-working-after-upgrading-t...) that finally solved the problem for me.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SharedAccess\Parameters
(here´s some open heart operation needed) - where you either have to change the Dhcp from .173. to .137 or depending on your release have to switch the scope addresses from .137. to .173. - MS did change something on the latest build (ending with .218)
Thanks for posting this link. Hopefully it will help some Windows 10 users resolving their mirroring and projection issues.