Unexpected binary element 255 when serializing

Question

I'm using Unity 2017 and wish to serialize save game data like so:

public void Save(Statistics data)
{
    BinaryFormatter bf = new BinaryFormatter();
    FileStream file = File.Create(Application.persistentDataPath
         + "/playerInfo.dat");
    bf.Serialize(file, data);
    file.Close();
}

The LevelData class is marked Serializable and contains only strings and ints. The class that implements IPersistentData meets those requirements too.

I did look for the problem and found that I should set an environment variable in my Awake method that I did, but that didn't solve anything. I'm using Windows 10 as operating system to develop on but the game will be made for Android.

When attempting to save, I always get this exception

SerializationException: Unexpected binary element: 255 System.Runtime.Serialization.Formatters.Binary.ObjectReader.ReadObject (BinaryElement element, System.IO.BinaryReader reader, System.Int64& objectId, System.Object& value, System.Runtime.Serialization.SerializationInfo& info)

Any sort of advice is welcome to get around the issue.

EDIT

Updating after trying the hints of Programmer: The class I wish to serialize:

[Serializable]
public class Statistics
{
    public int LastPlayedLevel;
    public Statistics()
    {
        MusicVolume = 1f;
        SFXVolume = 1f;
        LevelDatas = new List<LevelData>();
        LastPlayedLevel = 1;
    }
    public float MusicVolume;
    public float SFXVolume;
    public List<LevelData> LevelDatas;
}

And the LevelData:

[Serializable]
public class LevelData
{
    public LevelData(int levelNumber,string name)
    {
        this.LevelNumber = levelNumber;
        this.Name = name;
        this.BestPercent = 0;
        this.DeathCount = 0;
    }
    public int LevelNumber;
    public string Name;
    public int BestPercent;
    public uint DeathCount;
}