I have the same issue as others (see Login returns to login screen [closed]).
When I arrived at the login screen, I enter my password, the screen goes black for one second, displaying some text, and then it returns me to a fresh login screen.
I can go to a console alt+f1 and login into my account on the command line.
It is a fairly fresh install of ubuntu 12.04 64 bit, plenty of room left on the home dir. Permissions on the home directory are okay 755. Only one user.
Any hint because I cannot fix this apparently... Thanks!
Oh yes: the xsession-errors log says: "mkdtemp: private socket dir: Permission denied". Reinstalled unity, to no avail.