A rare example of good news:
I contacted my ISP, they contacted me back many times for additional
information and interacted with The Main Internet-Blocking Agency of
Russia (called Roskomnadzor), and bingo: the problem is solved. All 3
problematic sites:
https://www.gentoo.org/
https://devmanual.gentoo.org/
https://wiki.gentoo.org/
now work fine. Not only via my ISP, it's a global change.
It seems they wanted to block some "undesirable" sites in cdn77, but
blocked too much. Now they have fine-tuned what they block, and gentoo
sited are no longer blocked.
Andrey
