You are right, Moritz
Below is some lines of the syslog data which related to “named” service as your request:
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 06:47:40 dc-1 named[3510]: client 10.20.1.172#54101: update 'mydomain.xxx/IN' denied
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: cancelling transaction on zone mydomain.xxx
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.50-22426eb6.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.50-22426eb6.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.50-22426eb6.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 06:47:40 dc-1 named[3510]: client 10.20.1.172#55210: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 06:47:40 dc-1 named[3510]: client 10.20.1.172#55210: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: subtracted rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 06:47:40 dc-1 named[3510]: client 10.20.1.172#55210: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 06:47:40 dc-1 named[3510]: samba_dlz: committed transaction on zone mydomain.xxx
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 06:49:48 dc-1 named[3510]: client 10.20.1.172#54258: update 'mydomain.xxx/IN' denied
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: cancelling transaction on zone mydomain.xxx
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.50-22426eb6.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.50-22426eb6.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.50-22426eb6.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 06:49:48 dc-1 named[3510]: client 10.20.1.172#52240: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 06:49:48 dc-1 named[3510]: client 10.20.1.172#52240: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: subtracted rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 06:49:48 dc-1 named[3510]: client 10.20.1.172#52240: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 06:49:48 dc-1 named[3510]: samba_dlz: committed transaction on zone mydomain.xxx
Jul 27 06:50:02 dc-1 /USR/SBIN/CRON[10081]: (root) CMD (/usr/sbin/jitter 60 /usr/share/univention-samba4/scripts/sysvol-sync.sh >>/var/log/univention/sysvol-sync.log 2>&1)
Jul 27 06:50:02 dc-1 /USR/SBIN/CRON[10082]: (root) CMD (if [ -x /usr/bin/mrtg ] && [ -r /etc/mrtg.cfg ] && [ -d "$(grep '^[[:space:]]*[^#]*[[:space:]]*WorkDir' /etc/mrtg.cfg | awk '{ print $NF }')" ]; then mkdir -p /var/log/mrtg ; env LANG=C /usr/bin/mrtg /etc/mrtg.cfg 2>&1 | tee -a /var/log/mrtg/mrtg.log ; fi)
Jul 27 06:50:02 dc-1 /USR/SBIN/CRON[10092]: (root) CMD ( if [ -x /usr/sbin/univention-umount-homedirs ]; then /usr/sbin/univention-umount-homedirs; fi)
Jul 27 07:47:38 dc-1 dhcpd: Not configured to listen on any interfaces!
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 07:47:42 dc-1 named[3510]: client 10.20.1.172#56625: update 'mydomain.xxx/IN' denied
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: cancelling transaction on zone mydomain.xxx
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:42 dc-1 named[3510]: client 10.20.1.172#60445: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 07:47:42 dc-1 named[3510]: client 10.20.1.172#60445: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: subtracted rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 07:47:42 dc-1 named[3510]: client 10.20.1.172#60445: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 07:47:42 dc-1 named[3510]: samba_dlz: committed transaction on zone mydomain.xxx
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 07:47:46 dc-1 named[3510]: client 10.20.1.172#49630: update 'mydomain.xxx/IN' denied
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: cancelling transaction on zone mydomain.xxx
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:46 dc-1 named[3510]: client 10.20.1.172#60918: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 07:47:46 dc-1 named[3510]: client 10.20.1.172#60918: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: subtracted rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 07:47:46 dc-1 named[3510]: client 10.20.1.172#60918: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011AAAA#011fec0::8d2c:3b33:7b42:bd56'
Jul 27 07:47:46 dc-1 named[3510]: client 10.20.1.172#60918: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: subtracted rdataset mydomain.xxx 'mydomain.xxx.#01110800#011IN#011SOA#011dc-1.mydomain.xxx. root.mydomain.xxx. 2507 28800 7200 604800 3600'
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: added rdataset mydomain.xxx 'mydomain.xxx.#01110800#011IN#011SOA#011dc-1.mydomain.xxx. root.mydomain.xxx. 2508 28800 7200 604800 3600'
Jul 27 07:47:46 dc-1 named[3510]: samba_dlz: committed transaction on zone mydomain.xxx
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 07:47:49 dc-1 named[3510]: client 10.20.1.172#63515: update 'mydomain.xxx/IN' denied
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: cancelling transaction on zone mydomain.xxx
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: starting transaction on zone mydomain.xxx
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=AAAA key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: allowing update of signer=HDQ-TTBSP03L\$\@mydomain.xxx name=HDQ-TTBSP03L.mydomain.xxx tcpaddr= type=A key=1216-ms-7.51-22796573.9ea7728d-6d22-11e7-1fa9-5800e30b6b8b/160/0
Jul 27 07:47:49 dc-1 named[3510]: client 10.20.1.172#54464: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: subtracted rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011AAAA#011fec0::8d2c:3b33:7b42:bd56'
Jul 27 07:47:49 dc-1 named[3510]: client 10.20.1.172#54464: updating zone 'mydomain.xxx/NONE': deleting rrset at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: subtracted rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 07:47:49 dc-1 named[3510]: client 10.20.1.172#54464: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' AAAA
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011AAAA#011fec0::8d2c:3b33:7b42:bd56'
Jul 27 07:47:49 dc-1 named[3510]: client 10.20.1.172#54464: updating zone 'mydomain.xxx/NONE': adding an RR at 'HDQ-TTBSP03L.mydomain.xxx' A
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: added rdataset HDQ-TTBSP03L.mydomain.xxx 'HDQ-TTBSP03L.mydomain.xxx.#0111200#011IN#011A#01110.20.1.172'
Jul 27 07:47:49 dc-1 named[3510]: samba_dlz: committed transaction on zone mydomain.xxx
There is somthing wrong with the system which it deny the update to DNS database.