Do more on the web, with a fast and secure browser!
Download Opera browser with:
I've been having this for 1 year now. I have tried starting program with flag mentioned. Sadly this is not working. Most of my 15 plugins are disabled, the enabled ones are up to date. Also Opera is up to date. Neverthless, this message should be allowed to be disabled because what if user doesn't want to update. Huge deal breaker for me as this is really annoying with 14" laptop when using it when traveling. Bye Opera 
In opera://flags/ I disabled Async DNS resolver. Opera can still open the website. On Vivaldi or Edge, which are both based on chromium, site can’t be reached.
The domain which you can use to test and reproduce this is ohtuleht.ee. Visit it with Opera, then edit hosts file. Clear DNS cache of operating system and Opera browser. Still opens with Opera.
I have edited my hosts file and set a website to point at 127.0.0.1, I have cleared my DNS caches and opera://net-internals/#dns lookup validates that that website points to 127.0.0.1.
Those websites are blocked (redirected to be exact - connection refused) on Edge, Vivaldi as those browsers obey system DNS. I have visited that website before in Opera, meaning there is some cache somewhere and somehow it still opens even when domain is set to 127.0.0.1, it's loaded from a cloudflare server as it seems from network monitor.
I have deleted browser wide website data for all sites from settings but it is not helping. I kept the cookies as I need my logins and session kept.
My queston is how to make Opera to clear or flush it's DNS cache, as Clear host cache button on opera://net-internals/#dns does not change the situation and lookup of domain already hints that ip from hosts file is used.
Please mind the question asked, there is no need to reply with suggestion to use some kind of extension or question my way of doing things through hosts file.