so, I did some test again, the boat is at its berth, tiller position in the centre, turned on revers slowly - then led on server starts blinking slow, more speed gives more blinking. Also tried to reconnect the log transducer from/to NX2 server; did check cables nmea in and out; however no boat speed through water on Nexus instrument. As mentioned before a plotter is also connected to the system, so when C13 on multi instrument is on, the SOG is displayed. However if C13 is off, no boat speed.
Tried to take off log transducers and clean it, but it sits very tight, decided not touch unti