On Sun, 29 Mar 2015 at 13:46 Craig Sanders <cas@taz.net.au> wrote:
can anyone think of any reason why on one of my systems I can't login
with lightdm? as soon as I do (with any account, including a brand new
one without any weird/old/obsolete dotfiles), it seems to login OK but
then logs out again immediately.

Sounds like the xsession is dying. In typical fashion, errors will most likely get logged to $HOME/.xsession-errors, but not shown to the user.

I not entirely sure how lightdm works, but try looking in  $HOME/.xsession-errors after a dud login, and see if that helps.

Note this file will get overwritten if you login successfully, so you may have to use use a text mode console, or login as another user then sudo to root to access it.