This seems to only occur when (1)the file to be saved is selected in XYplorer and (2) the Preview Pane in XYplorer is open, thus showing the image. If I select another file before saving, I don't get the message."Could not save <filename> because the file is already in use or was left open in another application."
It also _usually_ only happens on the first attempt to save the file. After I clear the dialog box, the next attempt to save the file goes as expected. At least once, it happened several times in a row. I then selected a different image in XYplorer and the next attempt to save was successful.
At least once, I was unable to copy a file to another folder when its preview was being displayed. I can't seem to reproduce that issue right now, though. The Photoshop issue happens pretty much every time.
I'm pretty certain that this issue started with an update to XYplorer, not of Photoshop, late last year, although I can't pin down the exact point at which this changed. I do know that it didn't happen at all before that point, whenever it was. And it happens in both Photoshop 2020 and 2021.
I also tried this in Windows File Explorer, with its preview visible, and it doesn't seem to happen there.