Have Opera 45.0.2552.812 and running Ubuntu 16.04. VPN stopped working about an hour ago when VPN is enabled. When VPN is disabled Opera runs fine.
Any ideas on what happened? VPN was working fine until about an hour ago, the last few weeks.
The "Authentication credentials required." error happens from time to time. Seems to be a server-sided problem, so there is nothing you (we) can do on the client side. Usually VPN starts working again after a few hours of waiting without further interaction.
SurfEasy, the Opera VPN provider is having server issues and hopes to have them resolved soon.