NBXplorer: How to create a transaction? Can't get it verified

I’ve got some unspentCoins to different addresses some to Root ExtPubKey and some to derived ExtPubKeys. Now I want to spend it and can’t verify the transaction. My code looks like this:

var privateKeys = new List<ExtKey>();
var rootPrivateKey = GetRootPubKey(...);
var extRootPubKey = privateKey.Neuter();

privateKeys.Add(extRootPubKey);

var unused = await nbxplorer.GetUnusedAsync(userDerivationScheme, DerivationFeature.Deposit);
var maxUnusedNumber = unused.KeyPath.Indexes.Max();
for (UInt32 i = 0; i <= maxUnusedNumber ; i++)
{
  extPrivateKeys.Add(privateKey.Derive(i));
}

var changeAddress = await = nbxplorer.GetUnusedAsync(userDerivationScheme, DerivationFeature.Change);

var builder = new TransactionBuilder();
builder = builder
                    .AddCoins(unspentCoins)
                    .AddKeys(extPrivateKeys.ToArray())
                    .Send(destinationPubKey, amountMoney);
                    .SetChange(changeAddress)
                    .SendFees(new Money(fee, MoneyUnit.BTC))
                    .BuildTransaction(true);

verified = builder.Verify(tx);
// unfortunately verified is false

What did I do wrong?

Thank you in advance.