I found the solution. I post this because maybe someone else in the future has the same issue. The problem was with the library "qt5-styleplugins" from the AUR repository, so to solve the thing I had to rebuilt qt5-styleplugins with:
Code:
yay -S --rebuild qt5-styleplugins
I didn't think that an AUR-package can have this kind of impact on non-AUR-applications.