Can you further describe the issue?
Does the viewer cannot discover the primary machine anymore?
Or it can discover but unable to connect (with error info or display off)?
Please run the spacedesk Driver Console app in the primary machine, then click Diagnostics -> Save All Information button, then send us the diagnostic folder for further analysis.
PS: Saving all information will take a few seconds, then please send us the whole folder which contains all the diagnostic logs collected.
Please right-click the folder -> Send to -> Compressed (zipped) folder, then attach the *.zip file in your next reply.