"After upgrading to Windows 10, I find that the PrtScn button MEP 2016 Plus no longer brings up a dialog allowing me to save the maximized screen in the preview window as a jpg. Is there a workaround for this? I really want to capture the final screen of the video as a jpg without aspect ratio problems, and PrtScn used to do it."
After pressing PrtScn - check to see if the dialog is appearing underneath the MEP window.
. . . . I really want to capture the final screen of the video as a jpg without aspect ratio problems . . . .
If you want just an image of the final frame of the video you can use File, Export movie, Single frame as jpg - this will export in the same ratio as the video.
Sorry for the fire drill. A reboot (for some reason) restored the PrtScn function. And thanks for the tip about File|Export Movie|Single frame as jpg. Way cool...