I have now reinstalled the PC using Windows 10 64bit.
The only software
installed are OpenCPN 5.0 and Putty.
Still same problem. I need to start Putty, open serial port, close Putty and then start OpenCPN.
If I start OpenCPN first, the virtual serialport COM4 to YDNU-02 is not opening.
I have another serialport to a Miniplex-2s also, that (USB) serialport is working correctly all the time.