AFNetworking: Strange dealing with load balancing / server hosting / redirecting

Question

So, this issue is so hard to describe in short and I feel I should ask here first before raise an issue in AFNetworking github. Here's the story:

// Definition for words in the story.

Characters:
- APP: Our iOS app, which POST requests to a URL's API (has user input from a UITextField)
       APP's networking connection using framework AFNetworking 3.0
- DEV: Us developers, who created the iOS app.
- CUST: The troublesome customer, who live far from DEV.
        CUST used app before, then one day it doesn't work anymore (S1 has died).

Server:
- S1: The CUST's primary server (died for some reasons). This has the IP address = ip1
- S2: The CUST's secondary server (works fine). This has the IP address = ip2
- URL: The domain URL point to both server (using load balancing or something like that, I dunno).

// End-of-definition

Story begin:

CUST use app, enter URL, connection failed. CUST enter URL in iPad's browser, access normally. CUST ask DEV to check what happened. DEV enter URL in app, connect normally. At that time, DEV doesn't know that CUST has 2 servers.

Both CUST & DEV feel desperate, struggling for quite some times. CUST tried everything he can (used https / http, add / remove www, delete app and re-install). DEV got distract by unreasonable reasons (connector's error, api's error...). Then CUST remember that he's got 2 servers. CUST shared the IP addresses for DEV to check. That's when DEV find out that S1 has died but S2 hasn't.

So, DEV got the reason why CUST can't access to the URL using APP: the S1 has died. However, here's the real question for SO:

Why does it work in our end, but not in the CUST's end? Why APP (in DEV's hand) go to S2 but in CUST's hand, it go to S1 (and died)? But why CUST access URL using iPad's browser normally?

Does it has anything to do with DNS? VPN? Load balancing? I'd love to have a reason for this issue.

Thanks for reading. Feel free to edit this question, since I've also struggled to describe this as simple as possible.

UPDATE 1:

As James Jayson ask for how APP connect to URL, it use AFNetworking 3.0 with AFHTTPSessionManager with some setup:

self.sessionManager = [[AFHTTPSessionManager alloc] initWithBaseURL:baseUrl sessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
self.sessionManager.securityPolicy.allowInvalidCertificates = YES;
self.sessionManager.securityPolicy.validatesDomainName = NO;
self.sessionManager.requestSerializer.timeoutInterval = timeout;
self.sessionManager.requestSerializer = [AFJSONRequestSerializer serializer];

// Call API:
[self.sessionManager POST:path parameters:dictParams progress:nil success:^(NSURLSessionTask *task, id responseObject) { 
    // success block
} failure:^(NSURLSessionTask *task, NSError *error) {
    // failure block
}

UPDATE 2:

So, now I've got something new: CUST is using iOS 9.3.5, and DEV using iOS 10.x. DEV try to debug on iOS 9.3.5 device, CUST's error appear, error message be like this (almost the same, different with URL only).

LOG: Error Domain=NSURLErrorDomain Code=-1004 "Could not connect to the server." UserInfo={NSErrorFailingURLStringKey=APIURL, _kCFStreamErrorCodeKey=-2200, NSErrorFailingURLKey=APIURL, NSLocalizedDescription=Could not connect to the server., _kCFStreamErrorDomainKey=4, NSUnderlyingError=0x16146990 {Error Domain=kCFErrorDomainCFNetwork Code=-1004 "(null)" UserInfo={_kCFStreamErrorDomainKey=4, NSErrorPeerAddressKey={length = 16, capacity = 16, bytes = 0x100201bb253ce7160000000000000000}, _kCFStreamErrorCodeKey=-2200}}}

Now DEV is struggling with how to deal with this error. Please help.

Answer 1

Without knowing what the app is and how it works, I am guessing it is state-full and could be due to a session that was alive on the current dead server, the dev who starts a session is directed to server 2 via the load balancer.