Opera 40.0 is out and its VPN is working fine even without tweaking (at least, in my systems). So, you people can leave developer and switch to stable, if VPN is your primary reason for using the former

The reason you are getting "Authentication credentials required" is because you have not selected a country profile and the VPN location is set to Optimal...

I had tried that. It didn't work for me.

Try the following. It works for me.

Inside Internet Protocol Properties of your network connection: DO NOT obtain IP and DNS server addresses automatically. Assign valid IP and DNS server addresses.

If you're using Windows 7, right-click on the network icon in the system tray. Go to Open Network and Sharing Center. Right-click on the connection you're currently using. Go to Properties. Choose IPv4 if you're using it. Start from there.

And, good luck.

How do you set the maximum number of connections or downloads in Opera developer?