That domain resolves fine for my unbound installation.
SERVFAIL is just a generic error message indicating a server side resolution failure, making it somewhat hard to troubleshoot.
Before you dive into detailed unbound analysis as suggested in the topic linked by deHakkelaar, verify if you can access that domain now.
Ocassionally, SERVFAIL is seen only temporarily, e.g. because of intermittent high loads on a DNS server, or caused by a misconfiguration of an authoritative DNS server, which respective maintainers would have an interest in to address rather quickly.
If it's still not resolving today, a dig via Cloudflare's resolvers may return an Extended DNS Error (EDE) with an error code that is more detailed than the generic SERVFAIL (see MS Teams gets no presence status for contacts - #6 by Bucking_Horn for more details).
If none of the above is applicable, then analysing unbound in detail may at least allow you get a better understanding why that domain doesn't resolve.