The full error message usually also contains “There was no output. Response log file ”…“ was not created. This could indicate lack of write permissions to the log folder or problems starting WinSCP itself.”
This error indicates that WinSCP process started by WinSCP .NET assembly did not correctly start. Make sure your process has permissions to access WinSCP executable and to execute processes. See also related fix for for frequent use of the .NET assembly.