Here is the code how we fetch the state:
        List<IAuditableHandle> list = mRepository.itemManager().fetchAllStateHandles(workItem, mMonitor);
        List<IWorkItem> items = mRepository.itemManager().fetchPartialStates(list,
                    Arrays.asList(IWorkItem.STATE_PROPERTY, IWorkItem.MODIFIED_PROPERTY), mMonitor);
               
        for(IWorkItem w: items){
            result.put(w.modified(), mWorkFlowInfo.getStateName(w.getState2()));
        }

This code works fine for about 8000 defects, but fail for a single defect.

Because i'm unable to edit my comment.
The code will not fail in a exception. The result of one state is 'null'.

State in the context above has two different meanings
1. The state (version) of the work item 1st version created and saved, 2nd version created and saved etc.....
2. The workflow state

With your code, you won't see the workflow states as they are in the UI. The UI shows only the current state of the work item (and the current workflow state).

https://rsjazz.wordpress.com/2012/11/26/manipulating-work-item-states/ shows code around this.

Code to find the current workflow state of the current/latest state of a work item would look like

        int id = new Integer(idString).intValue();       
        IWorkItem workItem = workItemClient.findWorkItemById(id, IWorkItem.SMALL_PROFILE, monitor);

Yes, we want to see the history and all changes of the state for a single workItem.
The output is used for detailed reporting.
As i already mentioned this works fine for about 8000 workItems. But for one workItem, we see in the API call that some values are null.
Nevertheless the Webclient show always a valid value (in this case a String from the enumeration) for these timestamps.
We think this null values result from a DB2 problem and we want to fix this state on database level.

We know the workitem and the timestamp. We don't know the correct table to look for.