I use Siduction (Debian unstable) as distribution. In the last time there where no xfce-related updates and everything worked fine.
But now xfwm4 doesn't start while boot. xfce-session is working. I see my background picture, I can click on it and see the applications menu. I can open a terminal emu from there and start "xfwm4" manually. Than everything looks fine.
I can't find anything related to that with journalctl.
How and where is xfwm4 started normaly? Where should I look to do a diagnosis?
First try clearing your saved sessions cache. To be absolutely sure, delete the ~/.cache/sessions folder while not logged in. Its possible that your session was somehow saved without xfwm4 and on ever startup it is now excluded from starting. Clearing the sessions cache will restore the default startup components.
xfwm4 is usually started via xfce4-session.
That fixed the problem. Thanks.