First of all, I want to say thanks to your helping and I am really sorry, that I did not answer your comment, but I didn't have enough time to care about the bug itself. In the last days, I have tried all of I can do, so I have tried to install the latest version of Affinity Photo and install the newest drivers too, but they didn't help. Maybe good to know that I use a laptop that has a processor integrated graphical chip (Intel) and a dedicated Nvidia GeForce GT 520MX video card. Basically the PC use they alternately, and I have tried the different performance settings in the program too (only one of them or in WRAP mode), but nothing has changed.
Again I am really sorry for the late answer, but I had to concentrate on the university and other testes, so I hope you understand this. I don't know if I can do anything with this problem, but if you have any other idea, I would be really a pleasure for it.