WinSCP in web application - connect() fails

Question

I am working on a web application where one function is to connect to a remote Linux server using SFTP, download some log files, parse and display the results. I am using WinSCP to achieve this.

I got this working and tested it running from VS in my local dev box (Windows 10). I published it to a test server running Windows 2012 R2 and it fails to connect. It throws:

Timeout waiting for WinSCP to respond - WinSCP has not responded in time. There was no output. Response log file C:\Windows\TEMP\wscp3BFC.0292A297.tmp was not created.

I searched and it seems in IIS, impersonation had to be off and I had it on; that change did not fix the issue either although WinSCP support page lists it as a fix. Another WinSCP post mentioned that the executable had to be in same location as DLL or ExecutablePath should be used to specify where it is; I used that latter method.

When I remote to test server, open IIS, select the application and run it from there it works fine; so I know there are no issues with user ID, password, port or host key fingerprint.

This is the code I am using, in a class called SFTP:

private Session SFTP_Session;
private string SFTP_Host;
private int SFTP_Port;
private string SFTP_UserID;
private string SFTP_Password;
private string SFTP_SshHostKeyFingerprint;
private string ExePath;

public SFTP(string sHost, int iPort, string sUserID, string sPassword, string SshHostKeyFingerprint, string sExePath)
{
    SFTP_Host = sHost;
    SFTP_Port = iPort;
    SFTP_UserID = sUserID;
    SFTP_Password = sPassword;
    SFTP_SshHostKeyFingerprint = SshHostKeyFingerprint;
    ExePath = sExePath;
}

public void ConnectSFTP()
{
    // Set up session options
    SessionOptions sessionOptions = new SessionOptions
    {
        Protocol = Protocol.Sftp,
        HostName = SFTP_Host,
        PortNumber = SFTP_Port, 
        UserName = SFTP_UserID,
        Password = SFTP_Password,
        SshHostKeyFingerprint = SFTP_SshHostKeyFingerprint
    };
    sessionOptions.AddRawSettings("Cipher", "aes,chacha20,3des,WARN,des,blowfish,arcfour");
    SFTP_Session = new Session();
    SFTP_Session.ExecutablePath = ExePath;
    //SFTP_Session.XmlLogPath = "Z:\\Data\\WinSCP.xml";
    //SFTP_Session.SessionLogPath = @"Z:\Data\WinSCP.log";

    // Connect

    try
    {
        SFTP_Session.Open(sessionOptions);
        TraceLog.appendLog("Connected to " + SFTP_Host + ":" + SFTP_Port);
    }
    catch (Exception ex)
    {
        TraceLog.appendLog("Failed to connect to " + SFTP_Host + ":" + SFTP_Port + Environment.NewLine + "Error: " + ex.Message);
    }
}

It is called like this:

SFTP client = new SFTP(sftpHost, iSftpPort, sftpUserID, sftpPassword, sshHostKeyFingerprint, sExePath);
client.DownloadFiles();

public string DownloadFiles()
{
    ConnectSFTP();    // FAILS HERE ...
    try
    {
        using (SFTP_Session)
        {
            SFTP_Session.QueryReceived += (sender, e) =>
            {
            e.Continue();
            }
        };
        ....
    }
    catch (Exception e)
    {
        ...
    }
    finally
    {
        if (SFTP_Session != null)
        {
            SFTP_Session.Dispose();
        }
    }
}

I see in my log file (not WinSCP since it can connect):

2021-07-02 08:24:56:169|Failed to connect to 1.2.3.4:22 (ssh-ed25519-hK8U-......)
Error: Timeout waiting for WinSCP to respond - WinSCP has not responded in time. There was no output. Response log file C:\Windows\TEMP\wscp3BFC.0292A297.tmp was not created. This could indicate lack of write permissions to the log folder or problems starting WinSCP itself. 

WinSCP version is 5.17.10.0.