Hi Sebastian -
I think you've been watching too much TV with Mr. Trump, my name is Ronald, and people usually call me Edmund. :)
I haven't verified the qt bug on Ubuntu. If you have a link to an easy script, I can run it in the VM; AFAIK the Octave version which is now on "stable" is 4.02.
I would be more worried that we might have some numerical disparities between platforms; in fact we did some research at the Applied Maths Lab of Ecole Polytéchnique, and found that numerical disparities are to be expected between platforms and even software upgrades, for just about any code.
Edmund