PDF modified or corrupted after the document is signed [migrated]

I’m trying to add an external signature with iText 7.

I have implemented IExternalSignature this way:

 public byte[] Sign(byte[] bytes)
    {

        try
        {
            String HashToBeSigned = ComputeHash(bytes);

            string signature = here goes the call to a server, that gives back the signature  BASE64 encoded

            return Encoding.UTF8.GetBytes(Base64.Decode(signature));
        }
        catch (Exception e)
        {
            return null;
        }
    }

private String ComputeHash(byte[] message)
{
try
{

            SHA256Managed hashstring = new SHA256Managed();
            byte[] hash = hashstring.ComputeHash(message);
            string hashBase64 = Convert.ToBase64String(hash);
            return hashBase64;
        }
        catch (Exception e)
        {
            return null;
        }

    }

And the Call of signDetached is:
public static async Task SignPdfFile(String accessToken, String credentialId, String pin, String otp, String inPath, String outPath)
{
try
{
PdfReader reader = new PdfReader(inPath);
PdfSigner signer = new PdfSigner(reader, new FileStream(outPath,FileMode.Create), false);

            PdfSignatureAppearance appearance = signer.GetSignatureAppearance()
                    .SetReason("Reason")
                    .SetLocation("EN")
                    .SetReuseAppearance(false);
            Rectangle rect = new Rectangle(36, 648, 200, 100);
            appearance.SetPageRect(rect).SetPageNumber(1);
            signer.SetFieldName("sig");

            IExternalSignature pks = new CSCPAdESSignature(accessToken, credentialId, pin, otp);

          X509Certificate [] chain = await CSC_API_Utils.GetCertChainAsync(accessToken, credentialId);

           ICrlClient signingCertCrl = new CrlClientOnline(chain);

         List crlList = new List();
           crlList.Add(signingCertCrl);

          signer.SignDetached( pks, chain, crlList, null, null, 0, PdfSigner.CryptoStandard.CADES);
        }
        catch (Exception e)
        {

        }
    }

At the end, I get PDF modified or corrupted after the document is signed