Comment 1 for bug 779308

I guess the problem is that Auto-Login can't work together with an encrypted home directory.
You need to provide a password in order to unencrypt your home directory. If you do not provide a password than your home directory isn't accessible and kstartupconfig4 can't read e.g. ~/.kde_default/share/config/startupconfigkeys or ~/.kde_default/share/config/startupconfigfiles and fails.

I have a similar issue without auto login.

I regularly log in with password to kde. While logging in my Private ecryptfs directory is mounted. My .kde directory resides in ~/Private/.kde. The first login after startup always works and kde starts up fine.
When I logout of kde and try to relogin then I get the same error as javielillo. "kstartupconfig4 does not exist or fails. The error code is 3". The reason for this is that the Private directory isn't mounted after relogin. When I try to login from the console I get the error:
keyctl_search: required key not available
Perhaps try the interactive 'ecryptfs-mount-private'

This seems to be an issue with ecryptfs.