From https://bugzilla.novell.com/show_bug.cgi?id=238652 : I found out that the X server crashes when a client connects that uses opengl (was caused by overwritten libglx), simpler clients like xterm don't cause a crash. Ie the XOpenDisplay kdm does works and it assumes the server is running. Then it runs the greeter which crashes the server. In the ReapChildren function only crashes of the watcher process (I guess that -:0 thing) are checked for too frequent restarts. Crashes of the xserver itself are not counted.
KDM is unmaintained and not used in KDE Plasma 5. SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/