I wanted to make an new Admin profile and delete the old one (long story), but I forgot to set a password. Now I can only enter Guest session and the Administrator account is asking me about a password I haven't even set! What is the password that Ubuntu automatically sets for a new Admin profile?
You can boot with a live-cd and then mount the partition of your instalation with
sudo mount /dev/sda1 /mnt/
(/dev/sda1 ist an example you have to figure out where your instalation is)
Then you have to chroot in the mounted system with
you can set a new password for the root user.
Remember to unmount the filesystem when you are done
sudo umount /mnt