Azure table storage error: "Unexpected response code for operation : 99"

This un-descriptive error means that Azure bulk operations (at least in this case) takes up to 100 elements. Limit your batch and you'll be good.

I ended up using something like this:

public void Insert(IEnumerable<T> entities)
{
    foreach (var chunk in entities.Chunk(100))
    {
        InsertMaxLimitElements(chunk);
    }
}

private void InsertMaxLimitElements(IEnumerable<T> chunk)
{
    var insert = new TableBatchOperation();

    foreach (var entity in chunk)
    {
        insert.Insert(entity);
    }
    cloudTable.ExecuteBatch(insert);
}

The Chunk extension method was copied from this answer:

public static IEnumerable<IEnumerable<T>> Chunk<T>(this IEnumerable<T> source, int chunksize)
{
    while (source.Any())
    {
        yield return source.Take(chunksize);
        source = source.Skip(chunksize);
    }
}