I think I had also once on my computer.
But sorry I don't remember the cure anymore.

Try removing your Foxuser.dbf or 'zap' it.
Now I think i slightly remember what was wrong I work on a 2.6 and 6.0 project at the same time.
And by accident the foxuser.dbf 2.6 version ended up in my VFP project.
So by removing it Fox recreated it for me.
Just quit VFP.
From windows explorer delete the file and go back into VFP.
One drawback is that you will loose some of your preferences like debugger colors, edit window fonts, window positions.
But I think this is less worse than your mystery.