[mdlug] su Password problem

[Drew]

     The 'su root' command, and the 'Terminal Program - Super User 
Mode' application in the Terminals submenu,
have spontaniously stopped accepting my root password. Yast still 
accepts it, as does Administrator Mode in
Personal Preferences; and I can also log in as root from the Login 
screen. (I get a background wallpaper of
bombs and exclamation points.) But su returns "Wrong password".

     Why the different behavior for su? And why *did* su and the 
Super User Terminal app previously accept
my password?

     This is Suse 11.0, running KDE 3.5, Kernel 2.6.25.5-1.1-pae.

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   - Drew.