Session plugin: Does not restore any windows on restart of the system

As far as I understand this should be the purpose of this plugin:

It should "Talk to session manager and save/load window state", but I cannot get the plugin to achieve this.

Although there is a directory in home called .compiz/session and files are saved there, restoring those does not seem to work.

Also the files are small and do not contain all the info needed to restore opened windows, here just one window (class="Thunderbird" name="Mail" role="3pane" command="thunderbird") seems to be saved throughout the sessions...
But even this never got restored correctly.

The data saved for this window seems to be okay, just look at these 2 lines:
<geometry x="1920" y="24" width="1280" height="1000"/><maximized vert="yes" horz="yes"/>

Looks correct. But just there for one window, thunderbird...