can't resolve revenue.ie

[A. Schulze]

Hello,

we notices the name "www.revenue.ie" is un-resolvable via one of our  
unbound instances.
while looking for reasons we found no problem on a standby system.
unbound-1.8.1 configuration is *identical* (except listen ip)
The issue is reproducible even after restarting both instances.

neither http://dnsviz.net/d/revenue.ie/dnssec/ nor  
https://zonemaster.net/result/de1eaf5bb0b9a596
show (me) an obvious reason.

Some random tries later I configured "domain-insecure: revenue.ie"  
which solved the problem.

Cold someone explain that behaviour? Maybe we could learn again some  
more experiences debugging strategies ...

Thanks
Andreas