Hi Kloas,
Which version of VisualDMX do you use?
Can you see the firmware version of the RS-232 box in the options window (hardwware tab)?
The RS-232 -> DMX hardware is not compatible with Canvix (the RS-232 units were sold a long time before Canvix). If you would like start using Canvix then please consult your dealer, perhaps they can offer you an upgrade to a USB box like the Duple.