connect / as sysdba ORA-01031 Insufficient privileges

connect / as sysdba ORA-01031 Insufficient privileges

On a freshly installed 11gR2 Oracle Database, I set the following environment variables:

ORACLE_BASE
ORACLE_HOME
ORACLE_SID
TNS_ADMIN
TWO_TASK
LD_LIBRARY_PATH

However when I tried to connect to it:

sqlplus /nolog
conn / as sysdba
ORA-1031 Insufficient Privileges

A search on My Oracle Support showed this:

Error: ORA 1031
Text: insufficient privileges
-------------------------------------------------------------------------------
Cause: An attempt was made to change the current username or password without
the appropriate privilege. This error also occurs if attempting to
UPDATE a table with only SELECT privileges, if attempting to CONNECT
INTERNAL, or if attempting to install a database without the necessary
operating system privileges.
Action: Ask the database administrator to perform the operation or grant the
required privileges.

However I got my answer from this article:

UNIX: Checklist for Resolving Connect AS SYSDBA Issues [ID 69642.1]

It turned out that unsetting the environment variable TWO_TASK resolved the issue:

unset TWO_TASK
sqlplus /nolog
conn / as sysdba
Connected.