In order to fix the problem, I recommend you upgrade the software to the latest version. After that, go to Start > Control Panel > System and Security > Windows Firewall > Allow a program through Windows Firewall and check the box next to Download Accelerator Plus. If you can't find the application in the "Allowed programs and features" window, press the Allow another program button, navigate to the installation directory, select the executable file and press Add.
After that, from the Advanced Settings select Inbound Rules > New Rule. Check the Port box, click Next, select All local Ports and follow the self-explained steps. Also, make sure that your security program doesn't block the connection and right-click the launcher and select Run as Administrator.