So today it worked out - I just got a full debug log containing the time when the DNS gets broken.
As this contains plenty of personal data - is there something specific I can search for as I don’t want to post my data and also don’t want to spend hours anonymizing the logs?
edit: snippet of time frame in which DNS got broken:
2018-12-24 23:49:17 info kresd[26898]: > hints.add_hosts('/tmp/kresd/hints.tmp')
2018-12-24 23:49:17 info kresd[26898]: [result] => true
2018-12-24 23:49:17 info kresd[26898]:
2018-12-24 23:49:17 info kresd[26898]: > hints.add_hosts('/tmp/dhcp.leases.dynamic')
2018-12-24 23:49:17 info kresd[26898]: [result] => true
2018-12-24 23:49:17 info kresd[26898]:
2018-12-24 23:50:01 info /usr/sbin/cron[27171]: (root) CMD (nethist_stats.lua)
2018-12-24 23:54:00 warning kernel[]: [40819.695685] <[[--- VPN Traffic ---]]> : IN=eth1 OUT= MAC=d8:58:d7:00:43:00:38:10:d5:89:65:b2:08:00 SRC=112.78.45.158 DST=<router WAN IP> LEN=44 TOS=0x00 PREC=0x00 TTL=244 ID=22109 DF PROTO=TCP SPT=39007 DPT=80 WINDOW=14600 RES=0x00 SYN URGP=0
2018-12-24 23:54:30 warning kernel[]: [40849.551932] <[[--- VPN Traffic ---]]> : IN=eth1 OUT= MAC=d8:58:d7:00:43:00:38:10:d5:89:65:b2:08:00 SRC=110.249.212.46 DST=<router WAN IP> LEN=52 TOS=0x00 PREC=0x00 TTL=240 ID=0 PROTO=TCP SPT=55555 DPT=80 WINDOW=65535 RES=0x00 SYN URGP=0
2018-12-25 00:00:01 info /usr/sbin/cron[27429]: (root) CMD (nethist_stats.lua)
2018-12-25 00:00:01 info /usr/sbin/cron[27432]: (root) CMD (/usr/share/server-uplink/registration_code.sh)
2018-12-25 00:00:01 info /usr/sbin/cron[27435]: (root) CMD (/usr/bin/updater-supervisor -d --rand-sleep)
2018-12-25 00:00:01 info /usr/sbin/cron[27434]: (root) CMD ( /usr/bin/notifier)
2018-12-25 00:00:01 info /usr/sbin/cron[27436]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:00:02 info updater-supervisor[]: Suspending updater start for 1327 seconds
2018-12-25 00:01:01 info /usr/sbin/cron[27501]: (root) CMD ((cat /tmp/nethist.stats | sort ; echo "uptime = $(cat /proc/uptime | cut -d. -f1)" ) | logger -p info -t nethist ; rm /tmp/nethist.stats)
2018-12-25 00:01:01 info /usr/sbin/cron[27502]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:01:01 info nethist[]: fs_avg = nan
2018-12-25 00:01:01 info nethist[]: fs_max = 0
2018-12-25 00:01:01 info nethist[]: fs_samples = 0
2018-12-25 00:01:01 info nethist[]: fs_sum = 0
2018-12-25 00:01:01 info nethist[]: last_timestamp = 1545692399
2018-12-25 00:01:01 info nethist[]: load_avg = 0.0034014598540146
2018-12-25 00:01:01 info nethist[]: load_max = 0.17
2018-12-25 00:01:01 info nethist[]: load_min = 0
2018-12-25 00:01:01 info nethist[]: load_samples = 1370
2018-12-25 00:01:01 info nethist[]: load_sum = 4.66
2018-12-25 00:01:01 info nethist[]: mem_buffers_avg = 1824941.6408759
2018-12-25 00:01:01 info nethist[]: mem_buffers_max = 1841396
2018-12-25 00:01:01 info nethist[]: mem_buffers_min = 1817796
2018-12-25 00:01:01 info nethist[]: mem_buffers_samples = 1370
2018-12-25 00:01:01 info nethist[]: mem_buffers_sum = 2500170048
2018-12-25 00:01:01 info nethist[]: mem_cached_avg = 2424
2018-12-25 00:01:01 info nethist[]: mem_cached_max = 2424
2018-12-25 00:01:01 info nethist[]: mem_cached_min = 2424
2018-12-25 00:01:01 info nethist[]: mem_cached_samples = 1370
2018-12-25 00:01:01 info nethist[]: mem_cached_sum = 3320880
2018-12-25 00:01:01 info nethist[]: mem_free_avg = 1752392.6423358
2018-12-25 00:01:01 info nethist[]: mem_free_max = 1768924
2018-12-25 00:01:01 info nethist[]: mem_free_min = 1745480
2018-12-25 00:01:01 info nethist[]: mem_free_samples = 1370
2018-12-25 00:01:01 info nethist[]: mem_free_sum = 2400777920
2018-12-25 00:01:01 info nethist[]: mem_total_avg = 2070072
2018-12-25 00:01:01 info nethist[]: mem_total_max = 2070072
2018-12-25 00:01:01 info nethist[]: mem_total_min = 2070072
2018-12-25 00:01:01 info nethist[]: mem_total_samples = 1370
2018-12-25 00:01:01 info nethist[]: mem_total_sum = 2835998640
2018-12-25 00:01:01 info nethist[]: temp_board_avg = nan
2018-12-25 00:01:01 info nethist[]: temp_board_max = 0
2018-12-25 00:01:01 info nethist[]: temp_board_samples = 0
2018-12-25 00:01:01 info nethist[]: temp_board_sum = 0
2018-12-25 00:01:01 info nethist[]: temp_cpu_avg = nan
2018-12-25 00:01:01 info nethist[]: temp_cpu_max = 0
2018-12-25 00:01:01 info nethist[]: temp_cpu_samples = 0
2018-12-25 00:01:01 info nethist[]: temp_cpu_sum = 0
2018-12-25 00:01:01 info nethist[]: uptime = 41243
2018-12-25 00:02:01 info /usr/sbin/cron[27540]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:02:01 err server_uplink[]: Failed to get registration code
2018-12-25 00:03:01 info /usr/sbin/cron[27571]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:03:47 warning kernel[]: [41407.248121] <[[--- VPN Traffic ---]]> : IN=eth1 OUT= MAC=d8:58:d7:00:43:00:38:10:d5:89:65:b2:08:00 SRC=183.81.90.216 DST=<router WAN IP> LEN=44 TOS=0x00 PREC=0x00 TTL=49 ID=53708 PROTO=TCP SPT=3264 DPT=80 WINDOW=20811 RES=0x00 SYN URGP=0
2018-12-25 00:04:01 info /usr/sbin/cron[27602]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:05:01 info /usr/sbin/cron[27632]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:05:01 info /usr/sbin/cron[27633]: (root) CMD ( schnapps cleanup)
2018-12-25 00:06:01 info /usr/sbin/cron[27707]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:06:25 info hostapd[]: iot1: STA 2c:0e:3d:85:b9:92 IEEE 802.11: authenticated
2018-12-25 00:06:25 info hostapd[]: iot1: STA 2c:0e:3d:85:b9:92 IEEE 802.11: associated (aid 2)
2018-12-25 00:06:25 info hostapd[]: iot1: STA 2c:0e:3d:85:b9:92 RADIUS: starting accounting session 68C2C581784F3F44
2018-12-25 00:06:25 info hostapd[]: iot1: STA 2c:0e:3d:85:b9:92 WPA: pairwise key handshake completed (RSN)
2018-12-25 00:07:01 info /usr/sbin/cron[27738]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:07:36 warning kernel[]: [41635.574007] <[[--- VPN Traffic ---]]> : IN=eth1 OUT= MAC=d8:58:d7:00:43:00:38:10:d5:89:65:b2:08:00 SRC=104.131.136.175 DST=<router WAN IP> LEN=44 TOS=0x00 PREC=0x00 TTL=244 ID=54321 PROTO=TCP SPT=57955 DPT=80 WINDOW=65535 RES=0x00 SYN URGP=0
2018-12-25 00:08:01 info /usr/sbin/cron[27770]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:09:01 info /usr/sbin/cron[27800]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:10:01 info /usr/sbin/cron[27831]: (root) CMD (nethist_stats.lua)
2018-12-25 00:10:01 info /usr/sbin/cron[27833]: (root) CMD (/usr/bin/get-api-crl)
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'a.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'a.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'b.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'b.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'c.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'c.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'd.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'd.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'e.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'e.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'f.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'f.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'g.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'g.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'h.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'h.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'i.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'i.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'j.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'j.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'k.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'k.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'l.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'l.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'm.root-servers.net.', type: 1
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve address 'm.root-servers.net.', type: 28
2018-12-25 00:10:35 err kresd[26898]: [priming] cannot resolve any root server address, next priming query in 10 seconds