Thanks for all your comments and tips. I really learn a lot in this forum.
What I discovered as a solution for the PVA is, that since I reduced the printing speed of nozzle-2 drastically (to 35 in stead of 70), and left all the other parameters on Cura's default, that gave a fantastic improvement in the results. Although the total printing hours of my prototype increased by 9 hours!!
But it seems worth it.
Btw: I only use Ultimaker filament, and my PVA was brand new.
In Nozzle-1 I used nylon for the first time. Results are quite good.