su/kdesu not working

[Tom H]

>> I am sorry to have to post this problem to the list, but I got no answer in
>> the forum, and couldn't find one elsewere.
>> I can login as root.
>> when trying to su to root using "su" or "su -" i get "su: incorrect password"
>> following some threads I added my user to the "wheel" group and tried to edit
>> /etc/sudoers using visudo with no luck on both.
>> also tried to edit "/etc/pam.d/su" again with no luck.
>> I really need a solution since many operations can't be done without su
>> normally.

> Big hammer:
>  strace -f su -
> It will print a lot of stuff, maybe you will find something (in the
> last lines near the password rejection, if you are lucky).
> If some PAM module is involved, you may discover which one.

+1 for strace

And unfortunately, you can boot into single user without the root
password to change the root password.