[SalesForce] Approval lock/unlock with APEX

Why don´t you approve this process by quering the ProcessInstanceWorkitem, you can access the ProcessInstance Object to check for pending, rejected or approved requests and do specific actions on each case. Here is an example on how to approve one Item:

public static void approveApproval(Id aeId) {
        List<ProcessInstanceWorkitem> listaAprobacionPendiente = [SELECT Id, ProcessInstance.TargetObjectId
                                                                    FROM ProcessInstanceWorkitem
                                                                   WHERE ProcessInstance.TargetObjectId =:aeId];


        Approval.ProcessWorkitemRequest req = new Approval.ProcessWorkitemRequest();
        req.setComments('Aprobado desde proceso automático de aprobación.');
        req.setAction('Approve');
        req.setWorkitemId(listaAprobacionPendiente[0].Id);
        Approval.ProcessResult result = Approval.process(req);
}

I think you need to lock the record after insert or update. Not before. Try changing that to after event.

It does not make sense to lock a record before you have inserted it.

EDIT

Tested and works fine on API version 35