This is interesting...
See, I have Two monitors, always connected on a desktop PC. I also keep modal dialogues unstickied.
It is not unusual that if I open a file and Save as, export or similar, that the modal dialogue window appears on the other monitor than the one I am using, allowing me to still view my original file before committing changes. When this happens, the O.F. darkens (backdrop class). I wonder if your modal dialgue window is opening Off-Screen causing you to not be able to see it to proceed normally.
Doing a web search, I found:
Which led to: