I had the same issue, running Synaptic Package Manager from the menu was giving this error:

Code:
"unable to copy user's xauthorization file"
"unable to copy user's xauthorisation file" <-- added that for people who search for the international spelling of authorisation

I thought logging out of X and back in again would fix it, but that only got me stuck at the login screen as well. It then wouldn't log in again.

Ok so what was the problem and how did I fix it?

The problem appeared to be the permissions on the /tmp filesystem where many programs write their temporary files. My permissions for /tmp were 744 on my machine I think and I'm not sure how this happened.

To fix your permissions, if you're at the X login screen unable to log into X, then you will need to use a text terminal to set the permissions.

Hit Shift-F2 and log into the console.

Now fix the permissions of /tmp:

Code:
sudo chmod 777 /tmp
and enter your password when sudo asks for it.

Now programs can write to /tmp and hopefully this will resolve your issue. To exit from the terminal, type "exit" or just hit CTRL-D, does the same thing.

Now to get back to the X login screen, type Shift-F7 and you should be back at the gui, try logging in, see how you go.

Hope I could help people out there

Rock On!