When running a statement in the editor, such as select 'sysdate from dual;' an access violation error appears.
If it's run with F9 or F5, this happens. It works ok in SQL*Plus.
If an Oracle Forms 6i client is installed, as well as a normal 10g or 11g Oracle client, this will happen.
This is an issue with getting the decimal format from international versions of Oracle Forms.
Uninstall the Oracle Forms 6i client
Fixed in v11.5