I was annoyed that RubyMine's autocompletion did not work via Ctrl+Space for me. In fact, it did not work in any application.

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Turns out that keyboard combination was hijacked by Ubuntu as it's the default for switching input languages (i.e. keyboard layouts). If you use only 1 language/layout, you will not notice except for the key not working.

To fix it, do the following:

1. Run ibus-setup (e.g. from a terminal). This will open a GUI dialog.
2. In the 1st tab you should see "Next Input Method" followed by "<Control>space".
3. Click the "..." button on the right of it, and change it. I changed mine to "<Control><Shift><Super>space".
4. Click the "Apply" button, and then the "OK" button.
5. You may now close the iBus settings application.

Ctrl+Space should now work for you.