CryptographicException“未加载签名密钥”

我有一些代码试图使用 X509Certificate2

        byte[] bytes = Convert.FromBase64String(_loginDetails.CertificateAsString);

        X509Certificate2 cert = new X509Certificate2(bytes, _loginDetails.CertKey, X509KeyStorageFlags.Exportable | X509KeyStorageFlags.PersistKeySet);
        Reference reference = new Reference("#MsgBody");

        XmlDsigEnvelopedSignatureTransform env = new XmlDsigEnvelopedSignatureTransform();
        reference.AddTransform(env);

        SignedXml signedXml = new SignedXml(doc);
        signedXml.AddReference(reference);
        RSACryptoServiceProvider rsaKey = (RSACryptoServiceProvider)cert.PrivateKey;
        signedXml.SigningKey = rsaKey;
        signedXml.ComputeSignature();

        // Get the XML representation of the signature and add to the SOAP Header
        XmlElement xmlDigitalSignature = signedXml.GetXml();
        ...

此代码在8台不同的客户机器和我自己的机器上工作,但我们有一个客户出现错误:

签名密钥未加载于

有没有人更深入地了解为什么这会失败?证书和密码都正确,仅在客户服务器上出现故障。 .NET 4.5 .