So I've just migrated to Fedora Linux and ran over this problem when using my Idea. I have a notebook (no numpad) and I previously used the very same Idea (Ultimate, latest) with Win10 on this very same notebook and did not experience this.
I use default English layout. In the lower row of QWERTY, to the right there is / button which is also ? with shift. It works as expected anywhere else, but in Idea, when I use it as a CTRL + / shortcut it actually treats it as a Ctrl + . Thus, instead of commenting selected block of code, it collapses it. I checked it in Settings -> Keymap and this is exactly how Idea interprets it.
Is there any way to change this back to normal? I don't really want to rebind this hotkey as I'm used to it by now.
Posting a workaround that helped me. So the bug i'm experiencing is this one and it's weird how the bug is still there.
So in my case it didn't work as expected because I had Russian keyboard layout set as first. Switching Russian to second and English to first fixed the problem for me. That's not really a 'solution', rather than a 'workaround', but it worked for me.