Now I did remove the USB port from my Profilux and restarted to use the serial port on a Windows 7 machine, I had to change the device number direct on the Profilux back to 1.
After changing this number directly on the Profilux everything is working on Windows 7 using the serial port.
I asked to measure the USB port at work, but it seems that this interface is far to fragile for these power dips on the network, specially with the old type of lamps using a starter.

CONCLUSION

Changing the device number on the Profilux established the connection
USB is not completely usable. It will work but not all handlings will be go smoothly

I will try now to extend my serial cable, because my PC is not placed in my tank !!

Thank for the patience, to me this issue is closed.

Regards,
Hans