All I needed to do was to (read my own posting and) add the line
fObject.Close();
after the following one:
fObject.GetCOMPort(out strPortName);
So the complete foreach loop looks like this:
foreach (FTD2XX.FT_DEVICE_INFO_NODE node in arrInfoNodes)
{
if (node == null)
{
break;
}
else
{
if (fObject.OpenBySerialNumber(node.SerialNumber) ==
Ftdi.FTD2XX.FT_STATUS.FT_OK)
{
fObject.GetCOMPort(out strPortName);
fObject.Close(); // <<-- New Code here!
listResult.Add(strPortName);
}
}
}