DNS problem on one site today

I have an apache server on DO. It serves 6 simple websites (different domains). I haven’t changed anything for many years. Today, one of the sites is not working: Server Not Found. All the others are fine. The server is working fine (I restarted it, just in case), I don’t see any error in the logs. The registrar is namecheap, they quickly verified and answered that everything looks good on their side, it must be DO. DigitalOcean has not yet replied to my ticket after one day.

The DNS addresses are valid (ns1.digitalocean.com…) on both namecheap and DO.

ping syflex.biz ping: unknown host syflex.biz But ping works on all the other sites on the server.

dnschecker.org and whatsmydns.net say the site does not exist.

dig syflex.biz A +short doesn’t return anything, but it works on the other sites (that are on the same server) and return the expected: ns1.digitalocean.com. ns3.digitalocean.com. ns2.digitalocean.com.

dig syflex.biz returns SERVFAIL, but works fine with the other sites.

dig syflex.biz A +noall +answer doesn’t return anything, but returns the expected value on the other sites.

Same for: dig @ns1.digitalocean.com syflex.biz A +noall +answer

I downloaded the zone, it looks good, and similar to the other sites.

DO has an AI support. I spent 5 hours with it, without any result. It doesn’t feel safe to use it, as it returned several times wrong commands. It suggested to remove and re-create the A record, which didn’t help. It suggested to delete the domain and re-create it, which I find a little too drastic (it has several records for **DKIM, google-site-verification, etc).

Not sure what to do now.