PubkeyAuth disallowed for root only?

Hi folks,

we have a very weird phenomenon:

On two of our RedHat EL4 Update 4 servers root cannot login via ssh using public keys. All other users can. root is asked for a password instead.

sshd_config is RHEL4 default (that means pubkey auth allowed etc.), and the rest is as well. The public keys are correctly appended to /root/.ssh/authorized_keys. Rights on .ssh and its files are correct. I have even tarred+gzipped and copied roots .ssh directory to another server and tested there.

I have compared lots of things (sshd_config, pam config, even roots .bash_profile) to a reference system where it works - everything I have thought of is exactly the same.

PubkeyAuth has worked fine on the problem servers, but one of the admins must have changed something, and of course no one remembers anything. I am sure it is something small and silly I overlook.

Any hint and help is appreciated.