A 502 Bad Gateway error is usually a server-side issue rather than something caused specifically by Opera. It generally means that a gateway or proxy server did not receive a valid response from the upstream server hosting the site.
If the site was working in other browsers at the same time, I’d first clear Opera’s cache, disable any VPN/proxy or ad-blocking extensions temporarily, and try a private window. It may also be worth checking Opera’s DNS settings.
If the error appeared across multiple browsers, the problem was more likely related to the hosting server, reverse proxy, CDN, or a temporary outage. Checking the server/error logs would usually reveal the cause.
I’ve run into similar temporary gateway issues while working on a website, and in most cases the browser itself wasn’t the actual problem.