su doesn't work "Authentication failure"

Hello,

on my debian etch machine 'su' doesn't work if a password is needed. It is possible to do 'su someuser' from root but it's not possible to get back to root then using just 'su' or change from a normal user to another user account.

myserver:/tmp$ su
Password:
su: Authentication failure
Sorry.

auth.log says:
Jan 31 15:44:18 myserver su[27729]: (pam_unix) authentication failure; logname= uid=1000 euid=1000 tty=pts/4 ruser=myuser rhost= user=root Jan 31 15:44:20 myserver su[27729]: pam_authenticate: Authentication failure
Jan 31 15:44:20 myserver su[27729]: FAILED su for root by myuser Jan 31 15:44:20 myserver su[27729]: - pts/4 myuser:root

I suspect sth. in with the pam module got broken.

Any suggestions how to go on?

thx, paul

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