I have a TP600E with 256 RAM, I had this problem with 100% probabilty. I've changed from KDM to XDM and the problem is gone now, I can do shutdown and logout from XFCE. Unfortunataly oh so beautiful XDM VL login screen from the version 5.8 is gone and I cannot choose session with the butt ugly one provided with X. Or was it GDM? Anyways it doesn't seem to be in the repo.