Not sure exactly why this happens but it happens: I use Box cloud syncing software and my working project folder (which contains the blend file) is synced.
Box sync will not sync files flagged open until they are closed, so the main blend file is ok, it is synced when finally closed.
The versions though cause a problem: they must not be flagged and every save confuses the sync software and it starts making “user versions” of each version file, concatenating my user name to every blend1, blend2 etc and flooding my Windows UI with warning messages.
Can I set the folder where the version files are saved? (I have found in User Preferences the way to set how many versions, but not where…) I would then set a folder outside the synced area of my directory structure.