put a global messages component on your page and see what the message is
Things are clearer now... I've tried to track down the problem and the thing is: even though my a4j:support reRender target is the decorate around each input, the whole rest of the form is being validated as well. The error messages are related to the other fields in the form.
Strange thing is that I copied as is the xhtml structure from nestedbooking sample.
ajaxSingle on a4j:support did the trick but the thing is why it isn't specified on the examples and I guess there isn't that problem I faced?