I'm using quite a lot of PPA stuff so this could be the culprit but researched this one quite well and my conclusion is that this is related to Plasma Activities. This bug can on some systems be circumvented if the effect Present windows is disabled but appears to be only a temporary solution (as stated in bug reports I read). Logs provide no real insight and there is no Apport window.
Manually starting KWin after the crash works as expected. This bug doesn't appear if X Server gets restarted before login.
Update, as it appears this Plasma bug has a broader effect and crashes desktop composition if the Plasma desktop is set up to show independent widget sets on each virtual desktop or the Dashboard. Disabling the check for composition support makes this bug disappear.
Thank you for taking the time to report this bug and helping to make Kubuntu better. We are sorry that we do not always have the capacity to look at all reported bugs in a timely manner. There have been many changes in Kubuntu since that time you reported the bug and your problem may have been fixed with some of the updates. It would help us a lot if you could test it on a currently supported Kubuntu version. If you test it and it is still an issue, kindly upload the updated logs by running only once:
apport-collect 709421
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