I'm using Ubuntu 13.10 64bit. In the "System Settings"->"User Accounts" I chose the "Login without a password". This was my only administrator account, so according to this answer, I encountered bug #1168749, meaning I can't authenticate, not in the terminal and not in the GUI. I tried all of these answers:
After the first two I was again able to authenticate in the GUI, but still can't sudo in the terminal after trying all these answers and solutions.. Anyone knows how to fix this bug?
Actually I was wrong, I can sudo (silly mistake, my bad). I just kept trying to enter su and it said:
su: Authentication failure