migrating tru64 NIS server to linux

Tru64 managers:

We're retiring our Tru64 servers and migrating services to linux.
I'm trying to migrate the NIS service to linux and I can get clients
to login, but I cannot change the password with yppasswd - it simply
says: "Error while changing the password".

I'm only serving up the passwd, group and netgroup maps and I don't
have shadow passwords and the passwords are the crypt (des) hash values.

yppasswdd is running - I'm trying to change the password on the NIS master
server.  I wonder if it's some voodoo in pam or because I don't have
a shadow map ?

If anyone has any information/suggestions, I'd appreciate it.

Thanks,

Dirk