After the main WM session is finished, twm is launched with an xterm window. I believe, the problem is in the following lines in /etc/X11/Xsession:

        # Try default from runwm.
        /usr/X11R6/bin/runwm default

        # The system is broken...
        /usr/X11R6/bin/runwm failsafe

Two suggestions:
1) examine the return code of the \"runwm default\" command
2) replace runwm with a script that will detect a WM and return a command on standard output instead of running it.
This way, Xsession can exec the command.
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xinitrc-2.4.6-alt1