OT: howto debug

Hello,

my (multiple) unbound could not resolv "www.mywot.com".

I tried http://www.dnsviz.com and http://dnscheck.iis.se but found no obvious missconfiguration.
How can I dig in deeper to find the reason?

I found a difference, when using dig +trace:
$ dig www.mywot.com
...
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 46815

$ dig www.mywot.com +trace
...
www.mywot.com. 300 IN A 217.149.52.196
...

I also found, that the name is resolveble on systems which are ipv6 enabled.
unbound on v4 only systems could not resolv.

But no idea, what's really going wrong here.
Any hints?

Thanks.