... whenever I use the "copy selected photos" command.
I am using PM6 build 3954 (on Windows 10)
Could you please post a screenshot of the resulting empty window and one of your Copy/Move Photos dialog (that is what you're using to 'copy selected photos', right?)
Use the 'Attachments and other options' link when you're composing your reply to this message and there you'll be able to upload your JPEG format screenshots.
Thanks,
-Kirk