The site is working correctly now after I deleted a specific cookie from netatmo.com (and a few others likely unrelated). Makes me think if those cookies were even deleted properly when I used the non-specific "clear browsing data" function (cookies and site data, cached files).
It looks like the Win desktop version of Oprah browser isn't a priority for the dev team as they're probably focusing more on other things like their "gaming" browser (quite pointless IMO) etc.
I guess they can always shift the blame on the "ignorant" user utilizing older (58) version and so on, because in the most recent version everything works just fine, like the history and download tabs lol.
Have a nice day.
@leocg Thank you for the link.
I've just installed version 58.0.3135.132 and can confirm VPN indeed works here.
I guess I should've checked before downloading I picked the latest 58.* version installer and download it directly from Opera and not from some third party site which offered only v.58.*.127.
Have a nice and productive day.