https://community.zyxel.com/en/discussion/23595/why-i-get-bl...

Seems like you or someone upstream of you uses a Zyxel brand device that has some kind of dns content filtering enabled. You should be able to get around this on a given machine by configuring an alternate dns provider (dns over https, cloudflare's 1.1.1.1, google's 8.8.8.8, quad9's 9.9.9.9, etc.) or doing something similar at your own router/dns resolver/dhcp server if it's not the thing doing this.