To get the server working again I reinstalled the previous version of openssh and the service came up without complaint.

cd /tmp; rpm -i openssh-4.3p2-41.el5.i386.rpm --force --nodeps

cd /tmp; rpm -iv openssh-server-4.3p2-41.el5.i386.rpm --force --nodeps

cd /tmp; rpm -iv openssh-clients-4.3p2-41.el5.i386.rpm --force --nodeps

cd /tmp; rpm -iv openssh-askpass-4.3p2-41.el5.i386.rpm --force --nodeps

Lacking a session terminal window on that sever I used the command shell module of webmin to execute these commands after using its upload and download module to obtain the rpms from http://mirror.centos.org/centos/5.5/os/i386/CentOS/

I tried to report this problem to CentOS as a bug but the bug reporting service did not recognize my user id and password, notwithstanding I was logged into centos.org itself at the time.