I think I have found my mistake.
In the servlet, I had a reference (instance variable) to a stateful session bean. In the doPost/doGet method, I was accessing that bean (bean.getBlabla()). So when I was hitting my refresh button, the servlet doPost/doGet was called concurrently and therefore, the bean.getBlabla() was called concurrently as well. That is forbidden, as explained in section 7.5.6 of EJB 2.0 specification.