Whats this error message?

private void button2_Click(object sender, EventArgs e)
{
    SaveToFile(this.Controls);
}

private void button3_Click(object sender, EventArgs e)
{
    LoadFromFile(this.Controls);
}

private static byte[] key = Encoding.UTF8.GetBytes("MySecretKey1234").Concat(Encoding.UTF8.GetBytes(new string('0', 16))).Take(16).ToArray();
private static byte[] iv = new byte[16]; // 128 bit

public static void SaveToFile(Control.ControlCollection controls)
{
    string path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "configuration");
    if (!Directory.Exists(path))
    {
        Directory.CreateDirectory(path);
    }

    // File path and name
    string fileName = Path.Combine(path, "config.txt");

    // Create an AES object and set the key and IV based on the specified values
    using (Aes aes = Aes.Create())
    {
        aes.Key = key;
        aes.IV = iv;

        // Write the values of checkboxes and textboxes to the file, encrypted with AES
        try
        {
            using (StreamWriter sw = new StreamWriter(fileName))
            using (CryptoStream cs = new CryptoStream(sw.BaseStream, aes.CreateEncryptor(), CryptoStreamMode.Write))
            {
                using (StreamWriter swCrypto = new StreamWriter(cs))
                {
                    foreach (Control c in controls)
                    {
                        if (c is CheckBox)
                        {
                            CheckBox cb = (CheckBox)c;
                            swCrypto.WriteLine(cb.Name + "=" + cb.Checked);
                        }
                        else if (c is TextBox)
                        {
                            TextBox tb = (TextBox)c;
                            swCrypto.WriteLine(tb.Name + "=" + tb.Text);
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("An error occurred while saving configuration data: " + ex.Message);
        }
    }
}

public static void LoadFromFile(Control.ControlCollection controls)
{
    string fileName = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "configuration", "config.txt");

    // Create an AES object and set the key and IV based on the specified values
    using (Aes aes = Aes.Create())
    {
        aes.Key = key;
        aes.IV = iv;

        // Read the values of checkboxes and textboxes from the file, decrypted with AES
        try
        {
            using (StreamReader sr = new StreamReader(fileName))
            using (CryptoStream cs = new CryptoStream(sr.BaseStream, aes.CreateDecryptor(), CryptoStreamMode.Read))
            {
                using (StreamReader srCrypto = new StreamReader(cs))
                {
                    while (!srCrypto.EndOfStream)
                    {
                        string line = srCrypto.ReadLine();
                        string[] parts = line.Split(new char[] { '=' }, 2);
                        if (parts.Length == 2)
                        {
                            Control c = controls.Find(parts[0], true).FirstOrDefault();
                            if (c != null)
                            {
                                if (c is CheckBox)
                                {
                                    bool value = false;
                                    if (bool.TryParse(parts[1], out value))
                                    {
                                        ((CheckBox)c).Checked = value;
                                    }
                                }
                                else if (c is TextBox)
                                {
                                    ((TextBox)c).Text = parts[1];
                                }
                            }
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("An error occurred while loading configuration");
        }
    } 
}

Messagebox show

My fisrt question: Even if the error message appears, it writes and reads config.txt in the background. But why am I getting this error message? How do I update the codes? Where am I doing wrong?

My second question: SaveToFile is written in a single name here. What I want to do exactly is to create a new config file by randomly numbering the automatic config.txt in the specified file every time I save to file (For example: 123456789_config.txt, 123456788_config.txt). How can I fix for this?

My third question: While Open From File, it reads directly from the file itself. But I want to specify myself which file it should read through the File bower. How can I arrange?