WinSCP uploads file, but then deletes it from remote directory.

This problem started yesterday. I have been using winscp for years.
If I choose a file individually or choose "keep remote directory up to date", the pop up window shows the file(s) being uploaded, but then they are deleted from the remote directory.
Neither Winscp or my server is generating an error log.

I turned on confirmation of overwriting, and if the file already exists, it asks me to confirm, if it doesn't exist, it just uploads it, but in either case, the file is deleted immediately after

I'm quite sure that WinSCP does not delete the files. More likely some process was introduced on the server that picks the uploaded files for processing.

If you do not think that's the case, please attach a full session log file showing the problem (using the latest version of WinSCP).

You are right, my hosting provider decided to scan the server with a program that deleted any new files and put them in quarantine. Unfortunately it took them 12+ hours to figure this out.