This is not a caused by anything on our end; there is something in your browser settings that is blocking authentication cookies from being shared between subeta.net, sapi.subeta.net, and new.subeta.net. I am not familiar with what settings Opera gives you to manage cookies, but I know some browsers allow you to set strict settings that disable this sharing for security reasons. However, it is safe to allow all the *.subeta.net domains to share credentials in cross-origin requests. If you have changed any settings from the defaults, I recommend taking a closer look at them and see if anything of that sort is getting blocked. I think the default settings work since other Opera users do not have issues, so you could compare with the defaults and see if you can pinpoint the issue.

Oh you did reply here and I just didn't see it, sorry!

When you say you whitelisted subeta.net, do you know if that also whitelists all the subdomains? I would make sure that new.subeta.net and sapi.subeta.net are also included in the whitelist.

I disagree that it should be mentioned somewhere, since it is specific to how your browser settings work and we don't officially guarantee Opera support. If you (or anyone else) has further issues using unsupported browsers, just reach out like you did with this, and I am happy to help troubleshoot on a case-by-case basis when I am available. But there is also some responsibility on your shoulders to know what you're doing if you're using an unsupported browser and changing its advanced security settings.

I am not sure, but it's possible Firefox behaved differently because it also whitelists all subdomains when you whitelist a domain, whereas Opera appeared to have whitelisted only the exact domain you entered and not its subdomains.

Well, in any case, I'm glad I could help you resolve the issue :) Closing this up!