After upgrading an Ubuntu Gnome installation from 14.10 utopic to 15.04 vivid, the minimize and maximize buttons at the windows were missing.

This is how I got them back:

Run

dconf-editor

Then go to

org → gnome → desktop → wm → preferences

And change the value for “button-layout” from

appmenu:close

to:

appmenu:minimize,maximize,close

And close the window. That’s it 🙂