403 with initial API call when using Amazon Cognito iOS - ios

Question

I'm using Amazon Cognito for authentication and AWS iOS SDK v. 2.6.11 in my project. My app has the following flow on the main view: Get session, then make an API call using subclass of AWSAPIGateway class.
The issue here is that after successfully authenticating with Amazon Cognito, the API call response code is 403.
After stopping the app and then running it again (now the user is already authenticated) the response status code from the API is 200.
This is the message in responseData I get from the API call with 403 response:
"Message":"User: arn:aws:sts::############:assumed-role/####_unauth_MOBILEHUB_##########/CognitoIdentityCredentials is not authorized to perform: execute-api:Invoke on resource: arn:aws:execute-api:############:********####:##########/Development/POST/my-api-endpoint
(identifiers replaced with # characters)
It seems that the API calls are unauthorized. Is there a way to make those API calls authorized after an successful authentication?
This is the authentication code in my initial UIViewController:
let user = pool.currentUser() ?? pool.getUser()
user.getSession("myUsername", password: "myPassword", validationData: nil).continueOnSuccessWith { sessiontask -> Any? in
// i've left error handling out of this example code
let request = AWSAPIGatewayRequest(httpMethod: "POST",
urlString: "/my-api-endpoint",
queryParameters: nil,
headerParameters: nil,
httpBody: nil)
let serviceClient = AWSAPI_MY_AUTOGENERATED_Client.default()
return serviceClient.invoke(request).continueOnSuccessWith(block: { (task) -> Any? in
if let result = task.result, result.statusCode == 200 {
// A: all good - Continue
} else {
// B: Handle error (403 etc.)
}
return nil
})
This is how my AppDelegate looks like:
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
let pool = AWSCognitoIdentityUserPool.default()
let credentialsProvider = AWSMobileClient.sharedInstance().getCredentialsProvider()
let configuration = AWSServiceConfiguration(
region: .EUCentral1,
credentialsProvider: credentialsProvider)
AWSServiceManager.default().defaultServiceConfiguration = configuration
// keeping reference to the pool and the credentials provider
self.pool = pool
self.credentialsProvider = credentialsProvider
window = UIWindow(frame: UIScreen.main.bounds)
let rootViewController = MyInitialViewController()
window!.rootViewController = rootViewController
window!.makeKeyAndVisible()
return AWSMobileClient.sharedInstance().interceptApplication(application, didFinishLaunchingWithOptions: launchOptions)
}