Thanks you.
How do I get the required state identifier based on the state name?

Piotr, as so often in the API, you would have to get the workflowInfo, then the available states and then find the right state with a name matching what you look for. Look into the code in my post above. for a workflow you can use

IWorkflowInfo.getAllStateIds() and then IWorkflowInfogetStateName(stateId)

I came up with this

protected void setStatus(IWorkItem workitem, String statusName) throws TeamRepositoryException {
        IWorkItemClient workItemClient = (IWorkItemClient)teamRepository
                .getClientLibrary(IWorkItemClient.class);
        IWorkflowInfo workflow = workItemClient.findWorkflowInfo(workitem, null);
        Identifier<istate>[] states = workflow.getAllStateIds();
        for (Identifier<istate> state : states) {
            String name = workflow.getStateName(state);
            if (name.equalsIgnoreCase(statusName)) {
                workitem.setState2(state);
                return;
            }
        }
        throw new TeamRepositoryException("State " + statusName + " not found");

}