Cannot (consistently) save [Powerpoint] files [Edited]

It's very common, since my upgrade last year, to get this message: "The file cannot be saved right now because the file is loaded by another user at the moment. You can wait a few moments and try the save again, or you can cancel this save and resave using a different file name."
But there IS no other user. The file only exists on a single HD. It’s forcing me to save as in multiple versions. I’m up to 7!
how can this bug be fixed. Running OS XI on a Mac; Powerpoint 16.45
Max