Spurious (?) SERVFAIL (not-DNSSEC related)

I cannot get the name for 88.189.152.187:

# dig -x 88.189.152.187

; <<>> DiG 9.9.8-P4 <<>> -x 88.189.152.187
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 56967
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;187.152.189.88.in-addr.arpa.	IN	PTR

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Apr 18 06:24:57 UTC 2017
;; MSG SIZE  rcvd: 56

It is not a DNSSEC issue, I have the same problem with +cd:

# dig +cd -x 88.189.152.187

; <<>> DiG 9.9.8-P4 <<>> +cd -x 88.189.152.187
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 11278
;; flags: qr rd ra cd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;187.152.189.88.in-addr.arpa.	IN	PTR

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Apr 18 06:38:41 UTC 2017
;; MSG SIZE  rcvd: 56

I can reach the two authoritative name servers of 152.189.88.in-addr.arpa and they reply:

#  dig @ns2-rev.proxad.net. PTR 187.152.189.88.in-addr.arpa.

; <<>> DiG 9.9.8-P4 <<>> @ns2-rev.proxad.net. PTR 187.152.189.88.in-addr.arpa.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57772
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 2800
;; QUESTION SECTION:
;187.152.189.88.in-addr.arpa.	IN	PTR

;; ANSWER SECTION:
187.152.189.88.in-addr.arpa. 86400 IN	PTR	central.sources.org.

;; Query time: 9 msec
;; SERVER: 213.228.58.42#53(213.228.58.42)
;; WHEN: Tue Apr 18 06:39:20 UTC 2017
;; MSG SIZE  rcvd: 89

Where can I look for more details?

Knot DNS Resolver, version 1.2.5