There used to be a bug when using POST and clearing cookies on AFNetworking. Not sure what version you are using though. My guess is that it's still stored in AFNetworking and sent upon the next requests.
Have you tried clearing the cookies on the client-side (in AFNetworking?)
Something like:
NSArray *cookies = [[NSHTTPCookieStorage sharedHTTPCookieStorage] cookiesForURL: networkServerAddress];
for (NSHTTPCookie *cookie in cookies)
{
[[NSHTTPCookieStorage sharedHTTPCookieStorage] deleteCookie:cookie];
}