Notice: use 16 char length for both Key and IV, ex: Key: 1234567890123456 and IV: 9876543210654321
Here is C# code
    public byte[] Encrypt(byte[] someData, string KEY, string IV)
    {
        //preparing
        byte[] keyBytes = Encoding.UTF8.GetBytes(KEY); 
        byte[] ivBytes = Encoding.UTF8.GetBytes(IV); 
        //here goes encryption
        RijndaelManaged rijndaelManaged = new RijndaelManaged();
        rijndaelManaged.Key = keyBytes;
        rijndaelManaged.IV = ivBytes;
        rijndaelManaged.BlockSize = 128;
        rijndaelManaged.Mode = CipherMode.CBC;
        ICryptoTransform encryptor = rijndaelManaged.CreateEncryptor(rijndaelManaged.Key, rijndaelManaged.IV);
        byte[] result = null;
        using (MemoryStream memoryStream = new MemoryStream())
        {
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
            {
                cryptoStream.Write(someData, 0, someData.Length);
                cryptoStream.FlushFinalBlock();
                result = memoryStream.ToArray();
            }
        }
        return result;
    }
And here is AS3 code using AS3Crypto library
    private function decrypt(input:ByteArray, decrKey:String, decrIV:String):ByteArray
    {
        var key:ByteArray = Hex.toArray(Hex.fromString(decrKey));
        var pad:IPad = new NullPad();
        var aes:ICipher = Crypto.getCipher("aes-cbc", key, pad);
        var ivmode:IVMode = aes as IVMode;
        ivmode.IV = Hex.toArray(Hex.fromString(decrIV));
        aes.decrypt(input);
        return input;
    }