Clearing "Third party services data" under "Clear browsing data" fixed the "Authentication credentials required" when using Opera VPN problem for me:
Menu->More Tools->Clear browsing data
Uncheck everything except "Third party services data"
Select "the beginning of time" (I didn't check to see if a more limited date range would work)
Click "Clear browsing data"
@rob05667, Thanks Bro, this procedure worked for me as well, I think most people with this same problem would benefit
from "Clearing Browsing Data", Specially "Clearing Third party services data".
I even had another problem, namely I couldn't login to google.com because of an stupid error about not having cookies set and that is over as well, so thank you very much.