Flutter: Workmanager Backgroundtast Registration failing in IOS

Question

I am developing an app with flutter which requires to have repeatedly notifications (using local_notifications package). For Android everything is working properly with the code and the background task is working without an issue, independently whether the app is terminated or not. For iOS I am not able to register the backgroundtask successfully. I am always getting following feedback in Xcode: Could not schedule BGAppRefreshTask The operation couldn’t be completed. (BGTaskSchedulerErrorDomain error 3.)

So far I followed the iOS setup instruction from the ´workmanager´ package:

1. Adding lines to AppDelegate.swift
2. Adding task-identifier to info.plist
3. Register the background task in main.dart (working fine on android)

main.dart:

@pragma(
    'vm:entry-point') // Mandatory if the App is obfuscated or using Flutter 3.1+
void callbackDispatcher() {
  Workmanager().executeTask((task, inputData) async {
    print(
        "Native called background task: $task"); //simpleTask will be emitted here.
    try {
      if (await checkForJson()) {
        await displayNotificationWorkmanager();
      } else {
      }
      return Future.value(true);
    } catch (err) {
      print(err.toString());
      return Future.value(false);
      throw Exception(err);
    }
  });
}

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  // Workmanager().registerOneOffTask("task-identifier", "simpleTask");
  await flutterLocalNotificationsPlugin.initialize(
    initializationSettings,
  );  
  Workmanager().initialize(
      callbackDispatcher, // The top level function, aka callbackDispatcher
      isInDebugMode:
          true // If enabled it will post a notification whenever the task is running. Handy for debugging tasks
      );
  Workmanager().registerPeriodicTask(
    "NAME_NOTIFICATION_TASK",
    "notification_task",
    initialDelay: Duration(seconds: 10),
    frequency: Duration(minutes: 15),
    existingWorkPolicy: ExistingWorkPolicy.replace,
  );

  runApp(MyApp());

  // BackgroundFetch.registerHeadlessTask(backgroundFetchHeadlessTask);
}

Here is the AppDelegate.swift

import UIKit
import Flutter
import workmanager
import flutter_local_notifications


@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
 override func userNotificationCenter(_ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
                  completionHandler([.alert, .badge, .sound])
  }

    override func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        GeneratedPluginRegistrant.register(with: self)        
        // In AppDelegate.application method
        // WorkmanagerPlugin.registerBGProcessingTask(withIdentifier: "notification_task")
        
        // Register a periodic task in iOS 13+
        WorkmanagerPlugin.registerPeriodicTask(withIdentifier: "notification_task", frequency: NSNumber(value: 20 * 60))

        // FlutterLocalNotificationsPlugin.setPluginRegistrantCallback { (registry) in
        // GeneratedPluginRegistrant.register(with: registry)
        // }
        if #available(iOS 10.0, *) {
            UNUserNotificationCenter.current().delegate = self as? UNUserNotificationCenterDelegate
        }
        return super.application(application, didFinishLaunchingWithOptions: launchOptions)
    }

    override func application(
        _ application: UIApplication,
        performFetchWithCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void){
            WorkmanagerPlugin.setPluginRegistrantCallback{registry in GeneratedPluginRegistrant.register(with:registry)}
        completionHandler(.newData)
        }
    
}

Part of Info.plist:

<key>BGTaskSchedulerPermittedIdentifiers</key>
    <array>
        <string>notification_task</string>
    </array>
<key>UIBackgroundModes</key>
    <array>
        <string>fetch</string>
        <string>location</string>
        <string>processing</string>
        <string>remote-notification</string>
    </array>