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... see fix below.

After downloading the new Opera VPN version do the following:

1. Go to Opera "Menu" at top left of screen and then click "Settings"
2. Select "Privacy & Security"
3. Select the VPN option and select "Enable VPN" tick box
4. Close "Settings menu"
5. Open a new tab and you will see a small blue icon called "VPN" in the address Bar
6. Left click on the blue "VPN" icon
7. It will open a drop down menu showing a blue slider with the Option "VPN ON"
8. At the bottom of the drop down menu it will have a setting: "Virtual Location"
9. Beneath this it will have an option box which is set to "Optimal Location" (this is the default setting)
10. Click the down arrow option to change your location from the default (this is the cause of the problem)
11. Set the option to whichever country suits you best, you do not want it set as "Optimal Location"
12. Open a new tab and type in any address and ... Voilà!
    You do not have to change anything on your local PC or notebook as Opera browser is handling all calls using its own VPN ip address, enjoy surfing anonymously