Cause: The IDENTIFIED EXTERNALLY AS or
IDENTIFIED GLOBALLY AS clause was specified with a valid external name.
Action: Provide a valid external
name.

Relared Errors:
ORA-28026: user
with same external name already exists
ORA-28027:
privileged database links may be used by global users
ORA-28028:
could not authenticate remote server
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