AFHTTPSessionManager throws "Request failed: service unavailable (503)" error intermittently - ios

Question

This issue is intermittent and I have no idea why this is happening. Basically, I am trying to add a new post by sending a POST request to a remote RESTful API. I checked with POSTMAN that there is no problem with the API at all, but AFNetworking seems to fail some of its requests for no reason.
Here's my complete code:
- (IBAction)doneButtonPressed:(id)sender {
NSString *postBody = self.postInputField.text;
NSDictionary *params = #{#"postBody": postBody};
NetworkManager *manager = [NetworkManager sharedNetworkManager];
[manager POST:NEW_POST_API_URL parameters:params success:^(NSURLSessionDataTask *task, id responseObject) {
NSLog(#"NewPostSuccess. resp: %#", responseObject);
[[NSNotificationCenter defaultCenter] postNotificationName:ASK_TO_ADD_POST_ROW object:responseObject userInfo:nil];
} failure:^(NSURLSessionDataTask *task, NSError *error) {
NSString* ErrorResponse = [[NSString alloc] initWithData:(NSData *)error.userInfo[AFNetworkingOperationFailingURLResponseDataErrorKey] encoding:NSUTF8StringEncoding];
NSLog(#"%#", ErrorResponse);
}];
[self dismissViewControllerAnimated:YES completion:^{
[[NSNotificationCenter defaultCenter] postNotificationName:TABLE_SCROLL_TO_TOP object:nil userInfo:nil];
}];
}
and here is the error:
com.alamofire.serialization.response.error.response=<NSHTTPURLResponse: 0x170429b40> { URL: http://192.168.0.102:3000/api/posts/new } { status code: 503, headers {
Connection = "keep-alive";
"Content-Type" = "text/plain";
Date = "Mon, 05 Oct 2015 03:15:37 GMT";
"Transfer-Encoding" = Identity;
} }, NSErrorFailingURLKey=http://192.168.0.102:3000/api/posts/new, com.alamofire.serialization.response.error.data=<556e6578 70656374 65642065 72726f72 2e>, NSLocalizedDescription=Request failed: service unavailable (503)}
task: <__NSCFLocalDataTask: 0x136e6b150>{ taskIdentifier: 10 } { completed }
This happens intermittently, where I change nothing in the code and it starts working again in the same viewcontroller. The response error is useless as it only says unexpected error. Any guidance will be hugely appreciated.