Bug#491879: kwin does not launch automatically at starting an session
I found out, that there might be in the file
~/.kde/share/config/ksmserverrc the reason for this behaviour.
In this file there are some lines, like these:
I discovered, that in the non-working session, there is a comma (the ",") set
in front of the word "kwin" at the lines, were "kwin" is mentioned. How and
who this comma is setting, I could not find out, but if you are removing it,
everything is working fine.
I think, the coders know more about it.
I hope this helps !