The recurring 'your computer not known, please enter auth code' issue is solved for me.
I have static IP for business fibre, same public IP for over 2 years and a DHCP reservation internally. And yet I have received this multiple times.
This morning I disabled hyperthreading in order I could see if I could obtain greater throughput. As soon as I restarted and ran ESO, I got the 'PC not known....' message. I did NOT enter the code. I re-enabled HT and restarted, message went away.
Clearly a series of measures are being used to protect accounts, and this one is akin to MS Activation, any config change triggers an auth, not just a simple IP address change. This is a good idea, it's very unlikely anyone compromising your account is going to have the exact same config as you and so this is a high number of factors for auth, possibly better than 2FA with a token.