[Answer]-Ajax response with wrong callback function

From sencha docs about form.submit

success : Function

The callback that will be invoked after a successful response. A
response is successful if a response is received from the server and
is a JSON object where the success property is set to true,
{“success”: true}.

So your response should be a valid json and contain success:true like this:

{success: true, data: 'mydata'}