"log-servfail: yes" produce strange messages

[A. Schulze]

Hello,

recently I enabled "log-servfail: yes" and notice messages I find a little bit strange:

An empty cache receive it's first query for "andreasschulze.de. A" ...


2019-03-09 20:23:50.366270500 [1552159430] unbound[9110:0] info: start of service (unbound 1.9.1rc1).
2019-03-09 20:23:58.583770500 [1552159438] unbound[9110:0] info: generate keytag query _ta-4f66. NULL IN
2019-03-09 20:23:58.688137500 [1552159438] unbound[9110:0] error: SERVFAIL <m.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.688178500 [1552159438] unbound[9110:0] error: SERVFAIL <d.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.693592500 [1552159438] unbound[9110:0] error: SERVFAIL <i.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.693986500 [1552159438] unbound[9110:0] error: SERVFAIL <k.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.694066500 [1552159438] unbound[9110:0] error: SERVFAIL <g.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.700833500 [1552159438] unbound[9110:0] error: SERVFAIL <m.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.700897500 [1552159438] unbound[9110:0] error: SERVFAIL <i.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.723718500 [1552159438] unbound[9110:0] error: SERVFAIL <b.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.723751500 [1552159438] unbound[9110:0] error: SERVFAIL <j.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.734740500 [1552159438] unbound[9110:0] error: SERVFAIL <h.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.734771500 [1552159438] unbound[9110:0] error: SERVFAIL <f.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.734772500 [1552159438] unbound[9110:0] error: SERVFAIL <e.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.734860500 [1552159438] unbound[9110:0] error: SERVFAIL <c.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.739873500 [1552159438] unbound[9110:0] error: SERVFAIL <de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.743530500 [1552159438] unbound[9110:0] reply: 127.0.0.1 andreasschulze.de. A IN NOERROR 0.162199 0 62
2019-03-09 20:23:58.743934500 [1552159438] unbound[9110:0] error: SERVFAIL <de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.771714500 [1552159438] unbound[9110:0] error: SERVFAIL <k.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.771747500 [1552159438] unbound[9110:0] error: SERVFAIL <h.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.771994500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.775346500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.775506500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.778581500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.789290500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.792477500 [1552159438] unbound[9110:0] error: SERVFAIL <a.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.792532500 [1552159438] unbound[9110:0] error: SERVFAIL <e.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.792625500 [1552159438] unbound[9110:0] error: SERVFAIL <l.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.792651500 [1552159438] unbound[9110:0] error: SERVFAIL <j.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.811873500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.811963500 [1552159438] unbound[9110:0] error: SERVFAIL <denic.de. DNSKEY IN>: request has exceeded the maximum dependency depth (eg. nameserver lookup recursion)
2019-03-09 20:23:58.852893500 [1552159438] unbound[9110:0] error: SERVFAIL <l.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.852937500 [1552159438] unbound[9110:0] error: SERVFAIL <f.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.857556500 [1552159438] unbound[9110:0] error: SERVFAIL <d.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.861666500 [1552159438] unbound[9110:0] error: SERVFAIL <c.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.886935500 [1552159438] unbound[9110:0] error: SERVFAIL <g.gtld-servers.net. A IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.886969500 [1552159438] unbound[9110:0] error: SERVFAIL <b.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.
2019-03-09 20:23:58.886971500 [1552159438] unbound[9110:0] error: SERVFAIL <a.gtld-servers.net. AAAA IN>: could not fetch nameserver at zone gtld-servers.net.


how should I read this error messages? At lease https://zonemaster.net do not list any relevant problems for gtld-servers.net.de., de. or denic.de.
I'm a little bit confused if these messages hide other/true errors.

Andreas