Flashing this zip with twrp worked like a charm. Sorry for not having found the earlier post describing this problem & fix. Thanks a lot!!!
It seems that with DN4 v2 and v2.1 (I did not try v1) SideSync 3.0 is not working properly. Anybody else has the problem that the left mouse button does not function when mirroring the Smartphone-Screen on the PC-Monitor? Or anybody, where it does...