Ubuntu 18.04 - Screen doesn't to sleep; session doesn't lock

Question

Recently I discovered that my system doesn't let my screen go off, as in power saving mode. I have this setting right as I always did - the usual then 10 minutes.

What happens is that the screen dims and when it gets black it comes on again.

Also, I can't lock the screen manually or change user. It happens the same: screen dims and comes back where it was. In privacy settings I have it to lock.

Answer 1

Later on I found out that a Gnome extension was causing the issue. So I switched off one by one until I found the culprit. Problem solved!