Please check the instruction manual https://forum.spacedesk.netuser-manual/
Look at the chapters below
TROUBLESHOOTING
– Known Issues, Problems and Limitations
– Incompatible Third Party Products
TROUBLESHOOTING
– 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.