This error indicates that the primary machine can be reached over the network but connection fails after the indicated connection time.
Possible reason is an error on primary machine.
Is your primary machine Windows 10?
Is your graphics adapter Nvidia?
If yes, is Nvidia ShadowPlay (screen recording) currently running while you are trying to connect to spacedesk?
If yes, could you please try to switch it OFF then try again with spacedesk.
Current spacedesk has a known compatibility issue with Nvidia ShadowPlay technology.
Looking forward to your feedback.