MongoDB Realms: Unexpected Error in Java AndroidStudio

I am trying to make a simple connection to mongodb and insert some data. I am new to android studio and mongodb realms. I found some good and simple documentations which i also followed.
I followed these 2 documentations:

  1. https://www.mongodb.com/docs/realm/sdk/java/app-services/connect-to-app-services-backend/
  2. https://www.codersarts.com/post/connect-android-app-with-mongodb-using-realm
    I successfully connected to mongodb but now i get just a Unexpected Error. I don’t know what that means and have already checked some answers in the community but nothing helped.
    I also checked if i have my user wrongly formated but i think it is right.

I have really no clue what i can still do so any help is appreciated.
Here is my result i get in the console:

Down here I post all my code for connection to MongoDB and insert document

    MongoClient mongoClient;
    MongoDatabase mongoDatabase;
    MongoCollection<AppUser> mongoCollection;
    User user;
    App app;
    String AppId = "application-key";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Realm.init(this);
        app = new App(new AppConfiguration.Builder(AppId).build());
        app.loginAsync(Credentials.anonymous(), new App.Callback<User>() {
            @Override
            public void onResult(App.Result<User> result) {
                initializeMongoDB();
                AppUser appUser = new AppUser(
                        new ObjectId(),
                        "test",
                        "test",
                        "test@gmail.com",
                        "test",
                        22);
                mongoCollection.insertOne(appUser).getAsync(task -> {
                    if (task.isSuccess()) {
                        Log.v("EXAMPLE", "successfully inserted a document with id: " + task.get().getInsertedId());
                    } else {
                        Log.v("Example", "user:" + appUser);
                        Log.e("EXAMPLE", "failed to insert documents with: " + task.getError().getErrorMessage());
                    }
                });
            }
        });
    }

Than I have a function to call the function initializeMongoDB() where I connect to mongodb

    private void initializeMongoDB() {
        user = app.currentUser();
        mongoClient = user.getMongoClient("mongodb-atlas");
        mongoDatabase = mongoClient.getDatabase("database");
        CodecRegistry pojoCodecRegistry = fromRegistries(AppConfiguration.DEFAULT_BSON_CODEC_REGISTRY,
                fromProviders(PojoCodecProvider.builder().automatic(true).build()));
        mongoCollection = mongoDatabase.getCollection("user", AppUser.class).withCodecRegistry(pojoCodecRegistry);
    }

I define my user Object in AppUser class like this:

public class AppUser extends RealmObject {
    @PrimaryKey
    @RealmField("_id")
    private ObjectId id;
    @Required
    private String userName;
    @Required
    private String password;
    @Required
    private String email;
    @Required
    private String gender;
    @Required
    private Integer dob;

    public ObjectId getId() {
        return id;
    }
    public void setId(ObjectId id) {
        this.id = id;
    }

    public String getUserName() {
        return userName;
    }
    public void setUserName(String name) {
        this.userName = userName;
    }

    public String getPassword() {
        return password;
    }
    public void setPassword(String email) {
        this.password = password;
    }

    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }

    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }

    public Integer getDob() {
        return dob;
    }
    public void setDob(Integer age) {
        this.dob = dob;
    }


    public AppUser() {

    }

    public AppUser(ObjectId id, String userName, String password, String email, String gender, Integer dob) {
        this.id = id;
        this.userName = userName;
        this.password = password;
        this.email = email;
        this.dob = dob;
        this.gender = gender;
    }

    public AppUser(String userName, String password, String email, String gender, Integer dob) {
        this.userName = userName;
        this.password = password;
        this.email = email;
        this.dob = dob;
        this.gender = gender;
    }

    @Override
    public String toString() {
        return "[id=" + id + ", userName=" + userName + ", password=" + password + ", email=" + email + ", dob=" + dob + ", gender=" + gender + "]";
    }
}