The issue is actually a kind of good thing. The problem is that these sites have a messed up (or outdated) version of Cloudflare. If you open the console, you'll see:
61a79a69-cdb0-42d4-b801-ff5d9ae1c421-image.png
This is because Opera blocks cross-site requests to prevent sites from giving your data to other sites. Disabling your ad-blockers won't fix this issue.
The Cloudflare check works in Chrome. This does not mean that Chrome is less secure; it just means that Chrome trusts Cloudflare and made an exception.
Hopefully Opera will do the same soon, or maybe the sites will use new versions of Cloudflare!
In the meantime, there isn't much you can do. 😕
