Please check the instruction manual https://www.spacedesk.net/user-manual/
Look at the chapters below
– Known Issues, Problems and Limitations
– Incompatible Third Party Products
– WINDOWS PC (PRIMARY MACHINE)
– Incompatible Display Hook Driver Detected
Running Windows 10? Then the problem is usually caused by splashtop Wired XDisplay.
Uninstalling it will fix the problem.