www.makemkv.com

1.16.4 for Windows is able to decrypt a disc that the Linux version is not.

The Linux version's .MakeMKV/_private_data.tar file doesn't contain any hkd*.bin files, where the Windows version's does.

DNS is fine, net access is enabled on both.

Have you go through the steps listed in Fixing Volume Key is Unknown? Just because networking is enabled does not mean you don't have filtering going on. Certain DNS servers refuse to resolve some domains, and some firewall settings will block traffic from the Russian Federation (where the server is located).

Yes, except if I use exactly the same machine on Windows or Linux I get different behaviour. I can download from hkdata.fairuse.org fine.