@OctoL Without more clarity on which component is issuing that error we have to guess.
My guess is the error is coming from the Cloudflare API. Cloudflare itself is very reliable so the query from Let's Encrypt auth servers is not likely the one getting the SERVFAIL. And, as Osiris pointed out the status of your cert request says it never reached the point that LE checks for that TXT record.
I suggest
Review your Caddy Cloudflare config GitHub - caddy-dns/cloudflare: Caddy module: dns.providers.cloudflare
If that doesn't resolve it try posting at the Help - Caddy Community or maybe even the Cloudflare community
Or, even try using the HTTP Challenge in Caddy rather than Cloudflare. Or even the TLS-ALPN Challenge. Was there a particular reason you chose the Cloudflare and DNS Challenge? Automatic HTTPS — Caddy Documentation
I don't have much expertise in Caddy. Maybe a different volunteer here will offer more specific advice