I have encountered this problem about a year ago. It seems than passwd
only updates /etc/shadow when shadow passwording is enabled. If the root
password is disabled in /etc/passwd then running passwd won't help.
Try changing that line to
root:x:0:0:root:/root:/bin/bash
^
(If this solves your problem, it's a bug in passwd)
Best wishes:
Lorentey Karoly