Well, it was just a stock Hetzner dedicated server. All that was added/removed is a DNS AAAA record pointing to the server's IPv6 address. We used IPv6 before i

Well, it was just a stock Hetzner dedicated server. All that was added/removed is a DNS AAAA record pointing to the server's IPv6 address. We used IPv6 before in order to access LXC containers hosted on that server. If THAT counts as a broken setup (which I doubt, see below), then, sorry, I don't see how we could do better. I have personally checked the result from my then-home connection (unfortunately, through he.net tunnel) and it worked, and it was properly logged by Apache. The CEO had native IPv6 on the phone, and it also worked for him, and, according to Apache logs, IPv6 was also used. I have also checked that the site was still accessible over IPv4. So I indeed don't know what went wrong. On the other hand, I did not have access to Google webmaster console and had to trust the CEO that the SEO problem exists. It might have been a coincidence. And again, the SEO problem was in 2018, and is likely irrelevant today even if it really existed back then, that's why I said "in the past".