WP 20 has a weird bug. I configured it to save workspace on exit. When I exist and then return I get a message stating the files are already in use or in read only and to open with anew name. I went to Task manager on a whim and sure enough WP 20 is still in memory. I end task it and then restart WP and my saved workspace with the documents open up with no problem. Interestingly, I don't have this problem with WP 18.
I'm not sure but this problem in WP 20 cropped up when I installed Antidote, the French speller/grammar corrector. So there might be some kind of conflict between the two. I have the latest version of Antidote and WP 20.