json can not be analyzed on iOS and android side [on hold]

We are using a Slim framework of PHP to develop backend and we also have iOS and android application. Now we meet a problem that iOS- and android- end can not analyze JSON data.

iOS used AF Networking to analyze JSON and compiler gave us the message:

Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.}

JSON data got from android: {"stat":"1003","msg":"\u81ea\u52a8\u767b\u5f55\u4fe1\u606f\u5931\u6548\uff0c\u8bf7\u4f7f\u7528\u5bc6\u7801\u767b\u5f55","data":{"user_id":"","logo":"","name":"","token":"","balance":"","scores":""}}

But web-end could use JSON data normally.

Every time we've met the error we deleted all the PHP code and present the PHP code to the server, and the error no more existed. After some time we may meet the error again.

We've met this error many times, but couldn't resolve it. Thanks in advance.