Have you checked ownership and permissions? I've usually seen this error when there is a ownership/permissions mismatch but it could also be caused by no destiation directory. /usr/dt/config/Xsession.d would be where the XWindows information is located.
Do you have a /usr/dt/config/Xsession.d directory? - if not you can try creating the directory.
Try the ls -l command on the directories and make sure the onership/permissions are the same. If they aren't use the chown command to set them the same.
HTH
Mike