The default IBus key binding for "next input method" is Ctrl+Space, which prevents this key combination from reaching Emacs (or any other userspace program). To resolve the issue, run
ibus-setup and change the key binding for "next input method" to something else (or delete it entirely by clicking the "..." button and then the "Delete" button).
Note that running
ibus-setup from Unity, where
C-SPC works already, shows a preferences window that does not include the key binding option that needs to be changed.
The "next input method" command cycles through your configured inputs, e.g. perhaps from English to French.