RSA Encryption on Flutter and Decryption in Python

I am currently working on my first Flutter project and I would like to explain the situation first:

1. I am writing a Python Api using Flask which creates an RSA key pair as follows and stores them into files:

    def generate_rsa_key_pair(self):
    key = RSA.generate(2048)
    private_key = key.export_key()
    file_out = open("private.pem", "wb")
    file_out.write(private_key)
    file_out.close()
   
    public_key = key.publickey().export_key()
    file_out = open("public.pem", "wb")
    file_out.write(public_key)
    file_out.close()
   

2. The client, my Flutter app, now requests the public key to encrypt its messages with. This public key is then transmitted as follows:

    public_key = RSA.import_key(open("public.pem").read())
    keys["publicKey"] = public_key.exportKey("PEM").decode("utf-8")
   

   The public key that is stored in the file is also the same as the one in my Flutter app. I have already been able to verify this.

3. Now I want to encrypt a message in Flutter using RSA as follows. The public key is stored in the variable publicEncryptionKey:

    import 'package:encrypt/encrypt.dart' as enc;
   
    String asymmetricEncrypt(String message) {
   
      enc.Encrypter encrypter = enc.Encrypter(enc.RSA(publicKey: publicEncryptionKey,  encoding: enc.RSAEncoding.OAEP));
   
      enc.Encrypted msg = encrypter.encrypt(message);
   
      return msg.base64;
    }
   

4. I send the encrypted message back to my backend and now want to decrypt it with the private key. I do this as follows:

    from Crypto.Cipher import PKCS1_OAEP
    @app.route('/add-element-enc', methods=['POST'])
    def add_element_enc():
   
       values = request.get_json()
   
       if 'message' not in values:
          return "", 400
   
       value1 = values['message']
   
       private_key = RSA.import_key(open("private.pem", "rb").read())
       decrypter = PKCS1_OAEP.new(private_key)
       new_value = bytes(value1, 'utf-8')
       msg = decrypter.decrypt(new_value)
       return "", 201
   

My problem now is that decrypting does not work on my python backend and I get the following error when I try to decrypt the message:

ValueError: Ciphertext with incorrect length.

I'm getting worn out because I can't get it done. Is there anyone who already had a similar problem?