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Just started running into this issue with a number of Windows 11 Desktop PC's ..well similar ... if the monitor (or monitors) plugged into the Desktop PC is turned OFF - I get a grey screen when I connect via ScreenConnect ... and it's clickable, i.e. I can actually click the desktop elements of the client PC, just not getting the desktop showing to me on my end.

Reboot - doesn't fix it
Blank Guest monitor, then unblank it again - doesn't fix it
Re-installed video card (including my own nVidia 2060 at home!) - doesn't fix it
Ask someone to run over and physically turn the attached monitor on and BAM! ScreenConnect works fine. 

This is getting a tad annoying