Ctrl + shift + U combo does not work
In my case it was simple - Fedora 29 && Gnome 3.30.1
Just run: ibus-setup > Emoji Tab and change shortcuts for unicode code point and Emoji annotation to something different.
ibus-setup is shipped with package ibus-1.5.19-4.fc29.src.rpm
No need for restart nor logging out from current session.
I had this problem with Ubuntu 14.04 and Gnome classic...
Fixed it by going to Applications -> System Tools -> Language and support. On the Language tab, i switched "Keyboard input method" from "none" to "iBus". Logout/login or restart computer, and it worked like a charm :)