This isn't a password issue as the provided password is accepted.
As the error message states, it is a shell issue.

Possible causes are a broken root entry in /etc/passwd or a broken /etc/shells file.

You need to boot the system with CDROM and enter maintainance mode and do the following.

you should "boot cdrom -s"
then in the maintenance mode, mount /dev/dsk/cxtxdxs0 /a
vi /a/etc/shadow -> Change the shell of the root user to /bin/sh.
reboot the system

I guess it should work fine...

I am sorry....
In above post, wrongly i mentioned /etc/shadow file. But the corrcet one 'ld be /etc/passwd. Then execute "pwconv".