Hi Guys, today, rebooting, I was in trouble to login in my Gentoo system as root after, last time, a change to config files due to emerging some packages... in any way if You have problems with the graphical login window logging (KDE) as root, I hope this can be helpful solving it:
(or find the file 'kdmrc' in your system)
You can use Your preferred editor, in this case I used "nano", to open the "kdmrc" file looking at the path as shown above, so under "[X-*-Core]"
You must find "AllowRootLogin=false" so change it with "AllowRootLogin=true" and save.
Now if this was the problem then You've solved it, just reboot ;-)