That's a tricky one and it only happens on non-English keyboard settings.
When you set your keyboard to English the shortcut will always work because it is hard-wired in the keyboard interface.
Also the behavior is different depending on the keyboard layout: RSP-30422 - Toggle Comment Line works different depending on keystroke used
Although this problem exists since quite a time, we were not able to detect the trigger for that.
Well, it is sufficient to open the editor context menu to make it work again.