First time posting. Recently my WinSCP starts to upload 0 KB files (any kind of files) and I would like to share my thoughts. In my case it all comes down to SSL certs - while connecting without SSL, files are getting uploaded, but when using SSL (Lets Encrypt) WinSCP uploads 0 KB. If I'm using valid (even self-signed) SSL everything works fine, but with Lets Encrypt it fails. I tried the same with Filezilla and it handles situation the right way - it reports SSL cert (even expired), but connects anyway and uploads files as usual. I cleared cached host keys in my WinSCP, reconnect with SSL (valid Lets Encrypt cert just generated), dialog box appeared and confirms that SSL cert is valid, however WinSCP still uploads 0 KB files.
Thanks for your report.
Can you please post WinSCP session log file on level Debug 2 showing the upload both using self-signed certificate and Let's encrypt certificate? Verbose FileZilla log file showing a successful upload might be useful as well.