Hi,
I am experiencing the same problem,
I have changed the DNS server addresses several times, with no effect.
Sometimes reloading the page helps, sometimes not.
It seems that when I open the unaccessible page in Firefox (which always works) first, than the page can be opened in Opera as well. But this is not the procedure which I would like to do all the time...

I am using Opera 41.0.2353.46 on openSUSE Leap 42.1
Thanks