Thanks. Does your X11 log contain any additional info?
It's probably called:
/var/log/Xorg.0.log
or similar.
Are you trying to run QCAD as user 'root' or was that just for the strace?
Running GUI applications as root might not work as root might not have access to your X server.
If I remember correctly, you can give root access by running 'xhost +' prior to running QCAD.