do not put the a4j:log inside the h:form and/or re-rendered areas on the page.
Thanks for the advice but the ajax request in my code should not rerender the whole form but only the a4j:outputPanel inside it, right ?
I put the a4j:log element the same way as the sample code browsable in the demo: http://jsfunit.demo.jboss.com/jboss-jsfunit-examples-richfaces/richfaces/log.jsf, inside a h:form element.
I tried putting the element outside the h:form, it seems to work now. I suggest you change the example in richfaces demo.