I am currently uploading files to my Synology NAS.

If there are no files in the directory where you want to save the file, the file will be saved as I intended.

// ...
     ftpClient.enterLocalPassiveMode();
     ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
// ...
     ftpClient.storeFile(path, inputStream)

      public boolean storeFile(String remote, InputStream local) throws IOException {
           return this.storeFile(FTPCmd.STOR, remote, local);
      }

      private boolean storeFile(FTPCmd command, String remote, InputStream local) throws IOException {
        return this._storeFile(command.getCommand(), remote, local);
    }

   protected boolean _storeFile(String command, String remote, InputStream local) throws IOException {
        Socket socket = this._openDataConnection_(command, remote);
// ...
}


 protected Socket _openDataConnection_(String command, String arg) throws IOException {
        if (this.dataConnectionMode != 0 && this.dataConnectionMode != 2) {
            return null;
        } else {
    // ...
}

However, if there is at least one file, the this.dataConnectionMode value is set to 2 in the code above and null is returned.

What is this.dataConnectionMode and how can I solve it?