@yiclouis Sounds like a browser-sniffing error. That is to say, their website doesn't know what to do when it sees a person is using Opera. No current browser supports Java, so the stack trace is their website code.
The proper thing to do is complain to them. The altenative is to use a user-agent switcher to tell that website that your Opera GX is really Chrome or Edge. Telling them has the advantage of having them fix the site for all Opera users, though you have to wait for them to do so. Installing a user-agent switcher extension on the other hand is something you do yourself, no waiting for them. Of course, you could do both (which is usually what I do when I encounter such a site).