Can you create a reproducible test case? I'm 99% sure this is a problem with your backend not doing the right thing because we use the exact same proxy setup on our Twitter example and it works correctly.

It would also help if you paste the full response body that your server is sending back. It looks like you're sending back something like:

Code:
ExtdataJsonPcallback2({
    some: 'json'
});
When it should be:

Code:
Ext.data.JsonP.callback2({
    some: 'json'
});