Likely this issue:
Issue 2056 – File on local network share treated as folder when uploading

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I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

I solved my own issue.

The problem occurs when I try to move the files from a mapped drive on a Synology NAS. Because the drive is synced to my computer, I can go to c:/Users/(name)/SynologyDrive/(drive name)/folders/file and that works, but if I go to M:/folders/file it doesn't work.

I'm not sure why the "local" file (C: drive) works, but not the mapped drive, but glad to realize the issue and move on!

I've been doing my workflow the same way forever. Today, I received an Error Code 4, failure to create folder, when trying to transfer *.htm and *.xml files.

Windows desktop -> CentOS 7 server

I have write permission for the existing files on the server. I update them every day on my laptop, then transfer them to the server. The transfer works even though it returns an error code 3 (unable to set time/date) because I am in a group that has write permission.

This morning the transfer failed with the error code 4. I tried to transfer to a directory I have complete permissions for and it does indeed create a folder with the file name, then fails because there's no files (because it's not a folder, it's a file).

I can't figure out why today it wants to copy the files as folders...