I have no issues either, on .com, .uk, .de, .it etc. Funnily enough, I did get alerts from Avast earlier today about some cloudfront.com url it didn't like on .com but it's gone now. Site still worked though.
I guess that's how the modern web works - sometimes sh*t breaks, sometimes only for some users, using only some browsers, from some countries, and no-one else sees it and the web site fixes it silently, and no-one's the wiser what was going on.
My suggestion is to keep trying and make sure you do all the troubleshooting steps, there is something on your system or installs causing this. It does not happen for others.
https://help.vivaldi.com/desktop/troubleshoot/troubleshooting-issues/
One suggestion since you seem to be the technical type, is to use the Developer Tools, especially the Network tab, and see if you get any blocked requests or blocked cookies, stuff like that.
There is certainly nothing special about Vivaldi that would make it cause such issues.