thanks for your answer.
I watched the video, just that I get it right: According to my image above at the end I have a 240x240xK Image, were for every pixel there are K probabilities. The highest probability shows the class of the pixel right?
With regard to your second answer, that would mean that e.g. just use some kind of cross-entropy like: the truth should be [ 0 0 1 0 ] (class 3) but the fcn predicts [ 0.2, 0.8, 0.4, 0.1] right?