Andreas,

As I said above. It is RARELY the fault of the USB interface but more likely
the software that you are trying to use has not been programmed
using the proper methods to use serial ports within the windows
enviroment. I know this as when I developed by windows version
of the S8 ecu software, I had the same problems but learned how to do
it properly.

I advise you contact your software/product supplier and tell them.
It may get them to sort it for everyone. !