I had this problem today and I tried to backtrack what I was doing before I had this problem. I remembered I was customizing my desktop and change the settings for the number of bullets to show when I am putting my password to three dots for every character I put in. I configured it back to default - one dot per character and kdesu yast2 works again. Check it here...
Go to Menu -> Configure Desktop -> General Tab -> About Me. At the bottom, there's options for Password Prompts. Select "Shot one bullet for each letter". That fixed it. Hope it works for everyone too..

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