When it shows display off status on viewer app, did you check on your primary machine's display settings window if there's a detached second display there?
Can you also check the spacedesk SERVER window (just click the spacedesk tray icon), if the status shows ON (connected) or ON(idle) during display off?
For assistance, please also check our user manual. https://spacedesk.net/user-manual
For further analysis, could you please run dxdiag.exe on your primary machine, click "Save All Information", save the output file (dxdiag.txt) then attach it on your next reply together with your screenshots.
PS: Posting links are not allowed. "Attach" option will be visible below the text editor after your second post.