I took the time and made a few pictures to illustrate the story. The link to the images
http://www.andruskannel.planet.ee/shutter_vignet Sorry for the boring images. Just studio white wall.
I had not been able to generate problem with Elinchrom and skyport. My most used combo. Worked properly today. Sometimes I see very little black stripes with that combo too. So i took my first radio trigger system-Microsync Digital that I bought many years ago and where I was exposed to this problem for the first time and there they were, those black stripes. I tried to different cameras and all suffered the problem. Flash head I used was Bowens Esprit because Microsync need adapter to connect with Elinchrom.
And I did the test where I connected Microsync Digital trigger to camera sync terminal using the provided cable and that did not solve the problem. So, I can't blame the bad touch in hot shoe.
So my conclusin is that if you see those black stripes ( that is shutter) and you use radio trigger, then radio trigger causes the problem. I'm a photographer, not a physicist so that's why I can't explain why so. And the only solution that I know of - bad solution- use synk cable or optically- flash in hotshoe or good solution, buy a very good radio system.