1) Check that task parameter "Run with highest privileges" is disabled/unchecked
2) Set session.ExecutableProcessUserName = "<user that run the task>"
and
session.ExecutableProcessPassword = "user password"
as Windows SecureString

Can you set Session.DebugLogPath and post a log file of one of the dangling processes?
https://winscp.net/eng/docs/library_session#debuglogpath

Hey guys, I have an issue where several scripts are using Winscp to sftp upload or sftp download files. Everything works just fine for a day or so, then I get the errors "WinSCP process terminated with exit code 3. There was no output. Response log file C:\Windows\TEMP\wscp2578.0063037E.tmp was not created. This could indicate lack of write permissions to the log folder or problems starting WinSCP itself". This occurs for all the scripts.
I noticed in the taskmanager that they were over 100+ Winscp.exe background processes running, and started growing every time the scripts ran. If I killed them all the errors went away. At the end of all the scripts, I am disposing the sessions (session.dispose) and also setting the objects to nothing (e.g Set session = Nothing). I'm using version 5.13.8 (build 9278).