Ubuntu 18.04 Keystroke Alt + Shift + (Ctrl) + Down does not work

Gnome Tweaks helps me to fix it!

  1. Go to Keyboard & Mouse > Additional Layout Options.
  2. Turn off all options for "Switching to another layout".

    Gnome Tweaks layout switching options

In my case it luckily was just about re-mapping the keys in Settings->Keyboard->Shortcuts.

Here, moving the active window up/down was defined as Shift+Super+Page Up/Down, same without Shift was applied for moving one workspace up/down.

Re-mapping to what I was used to ([Shift+]Ctrl+Alt+Up/Down) did the trick :-)