Thanks for the good advice. Took me a while to read it all and try all the suggestions. Unfortunately, none of them worked.
I was able to boot in as root and enter and confirm a new password. That should have fixed it. But when I booted back into the openSUSE GUI, I continued to receive the "SU returned an error" message whenever I tried to get root privileges.
So I stopped trying and just reinstalled openSUSE. No more problem.
geoffro, I appreciate the benefit that unix password protection affords. For people who use a computer for important things whether at work or at home, it is foolish to try to bypass it. But I've been retired for over 20 years and use my computer strictly for fun. It is essentially a toy with which I play. There is absolutely nothing on it of any value, not even my real name. Without any data to lose or dependence on the machine for anything but enjoyment, entering the root password is just an annoying interruption. Especially in my case. I don't use my comp to play games or look at videos, etc. I test software and try to learn the computer's capabilities. So I require root privileges much more often than people who do useful things.
If I can't turn it off in openSuse, I'll probably go back to Mandriva or some other distribution that will let me, even (perish the thought) Windows.