I managed to fix it in the following way:
1. Install dconf-editor (to edit the "registry keys") sudo apt install dconf-editor
2. Open dconf-editor as a normal app; search for: /org/gnome/desktop/lockdown/
3. In my case, the disable-lock-screen was set turned on. I turned it off and everything now works fine!
I managed to fix it in the following way:
1. Install dconf-editor (to edit the "registry keys")
sudo apt install dconf-editor
2. Open dconf-editor as a normal app; search for: /org/gnome/ desktop/ lockdown/
3. In my case, the disable-lock-screen was set turned on. I turned it off and everything now works fine!