This is usually due to privilege problems or authentication issues on Windows (I presume your platform is Windows). I suggest reviewing the ORACLE_HOMEnetworkadminsqlnet.ora file and see if it contains a line like this:
If that line is present, remove it and try again.
Also make sure that the user you log on to the server with is in the local group named ORA_DBA. By default, the user that does the installation is placed in this group automatically.
If you are connecting to the server via terminal services or remote desktop services, then you should try going to the actual system console instead. Oracle has had problems with certain combinations of database releases and certain Windows versions.
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