If you're experiencing a "korgac" crash during login after the most recent kde/plasma updates, this is due to an issue with session management. I've logged a bug report with KDE - https://bugs.kde.org/show_bug.cgi?id=370883
Who knows how long before it's resolved. It appears that it might be related to QDBus, and Qt5 is still a full version behind Plasma. Perhaps we'll see a fix by Qt 5.8 - we're on 5.7.0 now, and 5.7.1 is in the pipeline before 5.8.
Meanwhile, there is a workaround.
Go to SystemSettings->Startup and Shutdown->Desktop Session->On Login and enter korgac
into the space entitled "Applications to be excluded from sessions". Save your changes.
The next time you login, you won't get the crash+coredump. Your xdg service will autostart korgac for you, anyway, so you aren't losing any functionality.
Also, please check your /var/lib/systemd/coredump directory for core dumps. You can delete these if you have no use for them (the vast majority of you won't have any use for them) - they do take up disk space.