Last tested: Apr 20, 2020
This is a generic error thrown by an SFTP server, it is very similar to the "RenderNoRender" error in that it's a catch-all rather than a specific error.
There are a few different things that can throw this, this article goes over some common causes to check: https://winscp.net/eng/docs/sftp_codes#code_4
Special considerations for Files.com SFTP Servers
For whatever reason, this is files.com's "access denied" and "path does not exist" error as well. So if they are using files.com, first check the path is correct and that the user in question has write permission to the destination folder. Note that with files.com, the path is relative to the default directory set for the user, so be sure to check that as well.