@leocg Yes. I do understand that it is not an Opera error message; it is a response from the website server. Nonetheless it only appears when I use Opera and does not appear when I use other browsers to access that website. So I am asking questions on a forum for the browser that seems to be dysfunctional in that situation.
I tried the suggestion from donq to clear cookies for this site only; I'm already running a VPN and shutting it off or on made no difference in this situation. After clearing the cookies, I shut down the browser and rebooted the computer (both of which failed to help in previous attempts to fix the problem. That worked; some cookie from that site was apparently responsible for the issue.
Thanks for your help.