@oberkut Yesterday I checked also Chrome and Chromium browsers, and there were no problem with them, so I don't think that the reason is site blocking by provider.
Currently this problem seems disappeared (at least I can open GitHub), but it's really strange because I've just turned on my desktop from hibernate, and there were no actions from my side after yesterday's troubles.

I confirm it for current versions of all editions: Opera Stable, Opera Beta, Opera developer. I've tried it all. Some sites couldn't be opened, for example github.com.