Seems that KDM cannot run any DE/WM except Plasma, Gnome-Shell, XFCE and LXDE. I fear this has been caused by http://quickgit.kde.org/?p=kde-workspace.git&a=commit&h=e7ce8adc506068491c8545a1990992267598bd33 See https://bugs.archlinux.org/task/40149 for more. Please ask if you need more info.
Andrea is right about the code region where the problem occurs, it's indeed in the lines added in above commit. Program terminated with signal 11, Segmentation fault. #0 0x000000000040d849 in startClient (pid=pid@entry=0x7ffffe8756a8) at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/client.c:1813 1813 /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/client.c: Datei oder Verzeichnis nicht gefunden. (gdb) bt #0 0x000000000040d849 in startClient (pid=pid@entry=0x7ffffe8756a8) at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/client.c:1813 #1 0x0000000000418756 in manageSession () at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/session.c:630 #2 0x0000000000411509 in startDisplayP2 (d=0x1545460) at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/dm.c:1500 #3 0x0000000000416d4f in startServerSuccess () at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/server.c:141 #4 0x0000000000408a13 in mainLoop () at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/dm.c:1289 #5 main (argc=<optimized out>, argv=<optimized out>) at /var/tmp/portage/kde-base/kdm-4.11.9/work/kdm-4.11.9/kdm/backend/dm.c:314
... and reverting the commit from comment 0 makes the crash go away.
Oops. Damn C code ;-) This should fix it: http://www.davidfaure.fr/2014/client.c.diff
Git commit e1724800ecf3c6a7035dfa7bcaa50b2a8f48688f by David Faure. Committed on 02/05/2014 at 08:26. Pushed by dfaure into branch 'KDE/4.11'. Fix crash when DesktopNames isn't set FIXED-IN: 4.11.10 M +7 -5 kdm/backend/client.c http://commits.kde.org/kde-workspace/e1724800ecf3c6a7035dfa7bcaa50b2a8f48688f
*** Bug 334721 has been marked as a duplicate of this bug. ***