The toggle comment shortcut is Ctrl + /, with the german keyboard layout this translates to Crtl + Shift + 7.
Everything worked fine until I upgraded last week to MinGW 5.1, Qt 5.5 and QtCreator 3.4.2
I don't know which upgrade is responsible.
If I change the layout to the english, the shortcut works perfect. But there is no need to press shift for a /.
But similar the increase font size shortcut is broken (in both layouts, but only in the english one it needs a shift).
Shortcuts which need directly a shift are working as expected.