[MusicKit] User Token request with Developer Token returns SKErrorCloudServiceNetworkConnectionFailed

We're building a small Unity/iOS application with an Objective-C plug-in to provide access to MusicKit. I'm going through the announcement talk which takes you through the process to access each component of MusicKit.

We've ticked off:

• Authorization to Media Library
• User Capabilities
• Getting Storefront Access code

Now we're onto the user token request and it's failing every time with the error SKErrorDomain Code=7, otherwise known as SKErrorCloudServiceNetworkConnectionFailed. The localised description written within the errors is 'Cannot connect to iTunes Store'. The final underlying error states that the HTTP status being returned is a 401 Unauthorized. See the full log of the response error here:

Error Domain=SKErrorDomain Code=7 "(null)" UserInfo={NSUnderlyingError=0x283058c60 {Error Domain=SSErrorDomain Code=109 "(null)" UserInfo={NSUnderlyingError=0x2830589f0 {Error Domain=SSErrorDomain Code=109 "Cannot connect to iTunes Store" UserInfo={NSLocalizedDescription=Cannot connect to iTunes Store, SSErrorHTTPStatusCodeKey=401}}}}}

I've found other responses to this particular error being cited as a bad developer token. This is potentially backed up as if I change the developer token in any way (replaced some random characters with 0s), I get the same response from the request, exactly as above with the same number of underlying errors.

However, if I use 'Postman' go through the process of sending requests through the Web API instead, using the developer token as a Bearer Authorization token, I get valid 200 OK responses from a request to, for example:

https://api.music.apple.com/v1/catalog/gb/songs/1161539476

If I alter the token in any way, this GET request returns a 401 Unauthorized response.

I've provided the relevant code below, redacted all irrelevant code. Any help would be really appreciated:

#import <Foundation/Foundation.h>
#import <StoreKit/StoreKit.h>

@interface MusicKitPlugin : NSObject
{
}
@end

@implementation MusicKitPlugin

static MusicKitPlugin *_sharedInstance;
SKCloudServiceController *_csController;
NSString *_developerToken;
NSString *_userToken;

+(MusicKitPlugin*) sharedInstance
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"Creating MusicKitPlugin shared instance");
        _sharedInstance = [[MusicKitPlugin alloc] init];
    });
    return _sharedInstance;
}

-(id)init
{
    self = [super init];
    if (self)
    {
        _csController = [SKCloudServiceController new];
        _developerToken = @"{INSERT_DEV_TOKEN}";
        _userToken = @"";
    }
    return self;
}

-(void)requestUserToken
{
    NSLog(@"Request user token called");
    [_csController requestUserTokenForDeveloperToken:_developerToken
                                   completionHandler:^(NSString *userToken, NSError *error)
     {
         NSLog(@"User token request responded");
         
         if(userToken != NULL)
         {
             NSLog(@"User token: %@", userToken);
              _userToken = userToken;
         }
         else if (error != NULL)
         {
             [self handleError:error];
         }
     }];
}

-(void)handleError:(NSError *)error
{
    NSLog(@"Error: %@", error);
    
    NSLog(@"Error domain: %@", [error domain]);
    NSLog(@"Error code: %ld", [error code]);
    NSLog(@"Error user info: %@", [error userInfo]);
    switch ([error code]) {
          ...
        case SKErrorCloudServiceNetworkConnectionFailed:
            NSLog(@"Cloud service network connection failed");
            break;
          ...
    }
}

@end

extern "C"
{
    void IOS_RequestUserToken()
    {
        [[MusicKitPlugin sharedInstance] requestUserToken];
    }
}

So I tap a button in Unity, that button calls the IOS_RequestUserToken method that is linked to in a MonoBehaviour in Unity from the Objective-C plug-in. That calls the requestUserToken function of the shared instance plug-in and it outputs:

Request user token called

User token request responded

Error: Error Domain=SKErrorDomain Code=7 "(null)" UserInfo={NSUnderlyingError=0x283058c60 {Error Domain=SSErrorDomain Code=109 "(null)" UserInfo={NSUnderlyingError=0x2830589f0 {Error Domain=SSErrorDomain Code=109 "Cannot connect to iTunes Store" UserInfo={NSLocalizedDescription=Cannot connect to iTunes Store, SSErrorHTTPStatusCodeKey=401}}}}}

Error domain: SKErrorDomain

Error code: 7

Error user info: {
    NSUnderlyingError = "Error Domain=SSErrorDomain Code=109 \"(null)\" UserInfo={NSUnderlyingError=0x2830589f0 {Error Domain=SSErrorDomain Code=109 \"Cannot connect to iTunes Store\" UserInfo={NSLocalizedDescription=Cannot connect to iTunes Store, SSErrorHTTPStatusCodeKey=401}}}";
}

Cloud service network connection failed