This is a known issue development are looking into. The issue being that by default the internal ‘nobody’ user has no permissions to modify graphs by default. Until this is fixed, it can be worked around by running the following SQL command (via iSQL or any SQL database connection, e.g., ODBC, JDBC, ADO.NET, etc.):
DB.DBA.RDF_DEFAULT_USER_PERMS_SET ('nobody', 7);