Cache.nixos.org: Couldn't connect to server (7) because of IPv6 from DNS

Hi. I suddenly started to see the following error:

$ nix-shell
warning: unable to download 'https://cache.nixos.org/dv9x2is9dcr10bzjfc823xbm59fwxb1p.narinfo': Couldn't connect to server (7); retrying in 255 ms
warning: unable to download 'https://cache.nixos.org/wwbfl8n274dndl7f01w1kjdy9silskc0.narinfo': Couldn't connect to server (7); retrying in 281 ms
warning: unable to download 'https://cache.nixos.org/dv9x2is9dcr10bzjfc823xbm59fwxb1p.narinfo': Couldn't connect to server (7); retrying in 521 ms
warning: unable to download 'https://cache.nixos.org/wwbfl8n274dndl7f01w1kjdy9silskc0.narinfo': Couldn't connect to server (7); retrying in 651 ms
warning: unable to download 'https://cache.nixos.org/dv9x2is9dcr10bzjfc823xbm59fwxb1p.narinfo': Couldn't connect to server (7); retrying in 1364 ms
warning: unable to download 'https://cache.nixos.org/wwbfl8n274dndl7f01w1kjdy9silskc0.narinfo': Couldn't connect to server (7); retrying in 1309 ms
warning: unable to download 'https://cache.nixos.org/dv9x2is9dcr10bzjfc823xbm59fwxb1p.narinfo': Couldn't connect to server (7); retrying in 2755 ms
warning: unable to download 'https://cache.nixos.org/wwbfl8n274dndl7f01w1kjdy9silskc0.narinfo': Couldn't connect to server (7); retrying in 2643 ms
error: unexpected end-of-file

curl reveals

$ curl -v https://cache.nixos.org/wwbfl8n274dndl7f01w1kjdy9silskc0.narinfo
*   Trying 2a04:4e42:9::729:443...
* Immediate connect fail for 2a04:4e42:9::729: Cannot assign requested address
* Closing connection 0
curl: (7) Couldn't connect to server

And Host

$ host cache.nixos.org
cache.nixos.org is an alias for dualstack.v2.shared.global.fastly.net.
dualstack.v2.shared.global.fastly.net has address 151.101.38.217
dualstack.v2.shared.global.fastly.net has IPv6 address 2a04:4e42:9::729

Does anybody know what is going on? Why does curl and other tools return IPv6 address of cache.nixos.org at first place?