Thanks for your suggestion. I have checked the layer view in Cura ... And, yes I found there that the misprint was already visible (no reason to start printing!).
I went back to my original design in Sketchup and found that in the pats were Cura showed the misprint, the object was having unlogical components. I changed that, exported to STL, reloaded in Cura and checked the layer view again: no errors!
At the moment I am printing again. I will let you know the outcome of that.