I upgraded wheezy today and with now xserver-xorg-video-radeon
1:6.14.4-2 the problem vanished.
The default kdmrc has an entry ...
# Restart instead of resetting the local X-server after session exit.
# Use it if the server leaks memory etc.
# Default is false
#TerminateServer=true
..., so the default "false" is working now.
Thank you!