Ubuntu – Dead keys (accents) stopped working after upgrading to Ubuntu 16.04

I was able to make it work by permanently disabling IBus with the following command (based on this answer):

im-config -n xim

Not sure whether IBus should be the default input method or not in new Kubuntu/KDE. So I don't know if this is more workaround or an actual fix (and IBus should not be used at all). Would be nice if someone with more in-depth knowledge can comment on that.

You can get rid of ibus package completely by sudo apt-get remove ibus (see comments).

Lucas Simon's solution worked perfectly in my Lubuntu 18.04.1 .

As I don't normally login as root, I opened an LXTerminal and ran the sudo variant of Lucas Simon's command line:

sudo apt install ibus && ibus-setup --xim

This quickly downloaded & installed some ibus packages, after which all my dead key accents immediately worked as intended - no further steps were needed, and I didn't even need to restart anything.