I was installing Diffeomorphic and foolishly installed Dropbox in the process. This somehow convince Blender that dropbox is the location for all my files. I got rid of drop box, but I can't convince Blender to use the file paths on my C: drive, or erase dropbox paths from its lists for importing and exporting, so these operations keep failing.
I uninstalled Blender and reinstalled it, but it preserved its memory of bad paths.
How can I fix this?