Comment 23 for bug 136529

I've commented out the three lines as suggested in #152577 but then all I get is:
/etc/gdm/Xsession: Beginning session startup
(session still lasts <10seconds...)

Adding "set -x" to the Default file:
...
+ echo /usr/bin/xsetroot
+ XSETROOT=/usr/bin/xsetroot
+ [ x/usr/bin/xsetroot != x ]
+ CHECKBACKCOLOR=OK
+ [ xTHEMED = xTHEMED ]
+ echo
+ sed s/^\([^ ]*\) .*$/\1/
+ CHECKBACKCOLOR=
+ [ x = xOK ]
+ BACKCOLOR=
+ [ x != xOK ]
+ [ x = xOK 1 ]
+ [ x = xOK 2 ]
+ [ x = x ]
+ BACKCOLOR=#dab082
+ /usr/bin/xsetroot -cursor_name left_ptr -solid #dab082
+ exit 0
/etc/gdm/Xsession: Beginning session setup...

putting the lines back I can confirm that this is what causes the setuid errors.
However it clearly isn't what causes the login problems! So I'll do some more digging...