Try using elgg.ajax instead of $.ajax

in http://urlpetition, do

json_encode($data);

forward(REFERER);

Set what that does

Thank you for your response. I investigated this issue a few more following your advises.

The function that generates the JSON response is a page handler, so the end of this function is:

echo json_encode($json);

If I put after this echo forward(REFERER), I receive in the second install my json encoded object plus a rendered page with an error message (forward after headers sent). In my first install I receive again this type of response:

{"output":{"success":true,"q":"14 menos 12"},"status":0,"system_messages":{"error":[],"success":[]}}

But if I put after this echo die(), I just only receive my json encoded object.

I know that this should be implemented in an action instead of a page handler, but I think that could be a bug of upgrading from 1.7.X to 1.8.1.

Make sure your page handler has return true;