Rijndael Encryption "Length of the data to decrypt is invalid." C#

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C#

encryption

Im creating an instant messaging service and whenever i want to decrypt data (except for the first message the client sends for some reason) it gives me the following error "Length of the data to decrypt is invalid.". I used a lot of the code from the microsoft website and in my dummy program it works perfect but for some reason my second message does not work properly

public byte[] RijEncryptString(string plainText, byte[] key, byte[] iv)
        {
            byte[] encrypted;

            using (Rijndael rijAlg = Rijndael.Create())
            {
                rijAlg.Key = key;
                rijAlg.IV = iv;

                ICryptoTransform encryptor = rijAlg.CreateEncryptor(rijAlg.Key, rijAlg.IV);

                using (MemoryStream msEncrypt = new MemoryStream())
                {
                    using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                    {
                        using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                        {

                            //write data to the stream
                            swEncrypt.Write(plainText);
                        }
                        encrypted = msEncrypt.ToArray();
                    }
                }
            }

            //returns the encrypted bytes from the memory stream
            return encrypted;
        }

        public string RijDecryptString(byte[] cipherText, byte[] key, byte[] iv)
        {
            string plainText = string.Empty;

            using (Rijndael rijAlg = Rijndael.Create())
            {
                rijAlg.Key = key;
                rijAlg.IV = iv;

                //creates a decryptor to preform the stream transform
                ICryptoTransform decryptor = rijAlg.CreateDecryptor(rijAlg.Key, rijAlg.IV);

                using (MemoryStream msDecrypt = new MemoryStream(cipherText))
                {
                    using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                    {
                        using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                        {
                            //read the decrypted bytes and put them in a string
                            plainText = srDecrypt.ReadToEnd();
                        }
                    }
                }
                //return the decrypted text
                return plainText;
            }
        }