This has started to happen after I encrypted my home folder (this was done after the installation, not as a part of it)... This was in 11.10.
I have since upgraded to 12.04 and after first reboot everything worked well. However since reboot after this, the messages started to pop up again before the actual KDM login prompt.
I am using Kubuntu 12.04 and I have tried different things such as permissions 777 for /tmp, removing everythign from /tmp or even deleeting ~/.kde or ~/.Xauthority etc.
Any ideas?
I had the same issue and found that my /home/USER/.kde folder was owned by root, which was causing permissions issues. After fixing the ownership issues with 'chown', things worked as expected.
Do this in terminal:
I could login after renaming my ~/.kde folder. Apparently something was corrupted and renaming the folder causes creation of a clean version. You will probably loose settings but at least you can login.
Logon with gnome, and try create another user, for some reason that I don't know, the config files for kde are invalid.
That worked for me.
I found this while searching for this very same problem when a RHEL7 VM that had worked for over a year, suddenly developed this problem. Every reboot and subsequent login as root displayed a small white popup stating: "Call to lnusertemp failed (temp directories full)". Other user accounts could login without a problem. I do not believe the problem is 'ubuntu'-specific based on this research.
With careful experimentation, I found that, for good reason, I'd recently added the following to /root/.bashrc: export TMPDIR=~/tmp
The problem with doing that, was /root/tmp did not exist. Either adding the target directory or commenting that line out immediately solved the problem on subsequent boots. Other testing proved that the same problem could be foisted upon plain user-accounts too.
So, either create the target directory, or don't put that in .bashrc for the applicable user. Good luck!