Finally, with the most recent driver version 3.4 from the chipset manufacturer (http://www.wch.cn/download/CH341SER_ZIP.html
) I got in touch with the controller.
I performed the PID optimisation via M303 S230 C5, saved the new settings via M301, and stored it to EEPROM by M500.
However, the temperature always stays below 235 °C floating around ca. 231 °C and printing doesn't start. The temp. controller goes briefly up to 235 °C but then switches off and the temperature dropps immediately.
What to do now?
So far, I'm not too amused about my Vertex nano, and I would expect some support from Velleman...