The first time I press PrtScr it generates a picture of the log-in background instead of the current window. The second time it hangs with the entire screen dimmed.
I have tried several combinations of Shift, Ctrl and Alt with the PrtScr button and when the guest reacts, it is when the host key is active. Every result was the same default background image rather than the current window, current workspace or even the current workspace background.
There are two saved images from several months ago of successful screenprints. I have been regularly updating the guest with all current security and general updates. This guest also gets re-booted several times a week.