Cause: During the course of opening a
privileged database link, the remote server was not securely identified using
the network security service. Additional errors should follow.
Action: Consult the network security
service documentation on how to properly configure the remote
server.

Relared Errors:
ORA-28029:
could not authorize remote server for user string
ORA-28030:
Server encountered problems accessing LDAP directory service
ORA-28031:
maximum of string enabled roles exceeded
ORA-28035:
Cannot Get Session Key for Authentication
ORA-28037:
Cannot Get Session Key for RACF Authentication
ORA-28038:
disallow O2LOGON
ORA-28039:
cannot validate Kerberos service ticket
ORA-28040: No
matching authentication protocol
ORA-28041:
Authentication protocol internal error
ORA-28042:
Server authentication failed