Some vendors still ship a version of RPC/NFS that allows at most 8 groups in the client credentials. Root on Solaris is by default in 10 groups. As a result, the Solaris 2.x mount command will send AUTH_UNIX credentials that are too big to cope with for the remote mount daemon resulting in the ``Invalid client credential'' error.
Workaround: put root and all your users in 8 or less groups. NOTE: You must logout and login again for changes in the number of groups to take effect. (or exit root's shell and re-su)