Hello,
the address of Japanese mirror KDDI has changed recently as follows.

KDDI R&D Labs (Saitama, Japan)
FTP: ftp://ftp.kddilabs.jp/Linux/packages/scientificlinux/
HTTP: http://ftp.kddilabs.jp/Linux/packages/scientificlinux/
HTTP: http://ftp.ne.jp/Linux/distributions/scientificlinux/
RSYNC: rsync://ftp.kddilabs.jp/scientificlinux/

So that, please edit URLs at
Downloading Scientific Linux: https://www.scientificlinux.org/download/

Regards, Green.