Just had the same issue, unable to login with any user in the GUI and only the root user via CLI.
When attempting to login to the GUI it would present an error "Login expired: Invalid cookie".
Solution was to login as root and check the filesystem usage with "df-h"
~ # df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda5 939M 566M 325M 64% /
devtmpfs 2.0G 0 2.0G 0% /dev
none 2.0G 244K 2.0G 1% /run
/dev/ram0 190M 50M 130M 28% /var
/dev/loop20 1.5G 1.4G 0 100% /tmp
/dev/sda7 939M 178M 712M 20% /tandberg
/dev/sdb2 220G 13G 197G 6% /mnt/harddisk
Based on the output it became apparent that the /tmp directory was 100% full.
Using the command "ls -lS /tmp" which displays the files in the directory sorted by size, it was easy to tell which file was taking up all of the space.
In my case it was the file "loggingsnapshot.txt" which is stored when you take a diagnostic log. This file had grown to over 1GB in size.
I removed the file with the command "rm /tmp/loggingsnapshot.txt" and then was immediately able to login again.
Hopefully this can help someone else in the future.
