Something blocks Ctrl+Alt+left/right-arrow keyboard combination

Ctrl+Alt+← and Ctrl+Alt+→ are default shortcuts for "switch to workspace left" and "switch to workspace right" respectively. You can verify that by running

gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-left

(and right in place of left).

Since by default GNOME shell has single column workspaces these shortcuts show no effects at all (unless you try something like this).

To unbind these keyboard combinations, run

gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "[]"

(similarly for right). Then you'll be able to use these combinations for your custom shortcuts.

The problem is that the Ubuntu 18.04 Settings GUI doesn't show all the keyboard shortcuts.

• It doesn't list "Move (Switch) to workspace to the left/right" (even though there is a shortcut assigned).
• It only shows the first keyboard shortcut if there are multiple shortcuts for a command, like for "Move (Switch) to workspace above/below", which actually has a second keybinding for Ctrl+Alt+up/down.

To see all of the window manager keybindings:

gsettings list-recursively | grep org.gnome.desktop.wm.keybindings | sort

org.gnome.desktop.wm.keybindings switch-to-workspace-down ['<Super>Page_Down', '<Control><Alt>Down']
org.gnome.desktop.wm.keybindings switch-to-workspace-left ['<Control><Alt>Left']
org.gnome.desktop.wm.keybindings switch-to-workspace-right ['<Control><Alt>Right']
org.gnome.desktop.wm.keybindings switch-to-workspace-up ['<Super>Page_Up', '<Control><Alt>Up']

Now, you can see that there are a bunch of Ctrl+Alt+up/down/left/right keybindings that weren't shown in the Settings screen.

To remove the unwanted keybindings:

gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-down "['<Super>Page_Down']"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-up "['<Super>Page_Up']"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "[]"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "[]"

Now you can use keybindings in other places.