If you go to Bluetooth and other devices in the Windows settings, find “Spacedesk Composite Device,” remove it, and reconnect the cable between your displays, then it should work.
This issue happened to me because I was updating spacedesk while my secondary device was plugged in. My assumption is that the only device that was actually disconnected was the virtual Graphics Adapter and the composite device remained active during the driver update. So when the update finished, I believe spacedesk didn’t recognize that the Composite Device was its own device and didn’t start the graphics adapter because of that.
