"Cannot move files to existing folder/file" - what does that mean?

I have a set of folders and files as user pages, generated by another script in what I believe to be correct form. (This is because I am migrating a non-Grav site.) When I try to modify the data from the admin page, when i try to save I get the message "Cannot move files to existing folder/file".

What does that indicate?

I can create new pages successfully from the admin plugin and I can modify and save those pages. I have not been able to find any differences between the pages created via the admin panel and those created by my external script. I have also checked the access permissions.