debian ssh su su: Authentication failure

The first thing to check is the permission on the 'su' command.

ls -l /bin/su

-rws--x--x 1 root root 55649 Apr 11 01:11 /bin/su

The SUID bit should be set (an "s" instead of an "x" in the first part of the permissions). If not, then log in as root and do a 'chmod +s /bin/su'.

chmod +s /bin/su