POST "base64 image string" not working in iOS 11

Question

Previously I'm saving "base64imagestring" on server using AFNetworking POST method and it's working fine but now from last couple of days i am facing some issues. I didn't change any thing in the request code but i am not able to upload base64imagestring on server.

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., NSUnderlyingError=0x281f25170 {Error Domain= example.com Code=-1011 "Request failed: unacceptable (406)" UserInfo=<NSHTTPURLResponse: 0x2811e4be0> { URL: http://example.com/Api/image_save.php } { Status Code: 406, Headers {
    Connection =     (
        "Keep-Alive"
    );
    "Content-Length" =     (
        226
    );
    "Content-Type" =     (
        "text/html; charset=iso-8859-1"
    );
    Date =     (
        "Wed, 29 May 2019 07:11:52 GMT"
    );
    "Keep-Alive" =     (
        "timeout=5, max=75"
    );
    Server =     (
        Apache
    );
} }}}}