Strange problem. After exception during commit (conflict with another thread) I get this exception. Scenario:

1. ThreadLocal pattern is used
2. Thread1 load myObject
3. Thread2 load myObject
4. Thread1 change some property of myObject
5. Thread2 add some object into some collection of myObject
6. Thread1: session.flush() and commit()
7. Thread2 session.flush() and commit - correct Exception is thrown !

8. Thread3 (!) load myObject - Exception null index column for collection is thrown

I must restart server (oc4j). Database is OK and after restart of server, application is OK.