Hi Spacedesk Lea,
I have the same case with the same issue.
I tried to use my iMAC 27″ (2017) with a Radeon Pro 570 4096 MB as a secondary screen for my windows Laptop. Therefore I installed Windows 10 (via bootcamp) on my iMAC and did everything like Schneider97.
I also have an established connection and can take over control but the (viewer)screen remains black
Could you please help?
Many thanks in advance