I have no further occurrences of NsReleaseLock in older log files.
[root@myserver dovecot.conf]# grep NsReleaseLock /var/log/messages-*
[root@ myserver dovecot.conf]#
[root@duplex dovecot.conf]# grep nethserver-check-nsreleaselock /var/log/cron*
/var/log/cron:Dec 2 08:33:38 myserver run-parts(/etc/cron.daily)[5447]: starting nethserver-check-nsreleaselock
/var/log/cron:Dec 2 08:33:39 myserver run-parts(/etc/cron.daily)[11366]: finished nethserver-check-nsreleaselock
/var/log/cron:Dec 3 05:26:57 myserver run-parts(/etc/cron.daily)[24034]: starting nethserver-check-nsreleaselock
/var/log/cron:Dec 3 05:26:58 myserver run-parts(/etc/cron.daily)[11029]: finished nethserver-check-nsreleaselock
/var/log/cron:Dec 4 04:04:58 myserver run-parts(/etc/cron.daily)[5880]: starting nethserver-check-nsreleaselock
/var/log/cron:Dec 4 04:06:07 myserver run-parts(/etc/cron.daily)[22308]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 4 05:59:10 myserver run-parts(/etc/cron.daily)[6509]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 4 05:59:11 myserver run-parts(/etc/cron.daily)[21602]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 5 08:31:11 myserver run-parts(/etc/cron.daily)[5461]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 5 08:31:12 myserver run-parts(/etc/cron.daily)[30499]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 6 06:52:50 myserver run-parts(/etc/cron.daily)[6208]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 6 06:52:51 myserver run-parts(/etc/cron.daily)[30177]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 7 05:57:17 myserver run-parts(/etc/cron.daily)[32215]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 7 05:57:18 myserver run-parts(/etc/cron.daily)[30963]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 8 04:14:19 myserver run-parts(/etc/cron.daily)[18339]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 8 04:14:20 myserver run-parts(/etc/cron.daily)[25756]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 9 08:12:05 myserver run-parts(/etc/cron.daily)[1070]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 9 08:12:06 myserver run-parts(/etc/cron.daily)[17962]: finished nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 10 04:23:34 myserver run-parts(/etc/cron.daily)[31648]: starting nethserver-check-nsreleaselock
/var/log/cron-20181111:Nov 10 04:23:35 myserver run-parts(/etc/cron.daily)[6636]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 11 06:09:08 myserver run-parts(/etc/cron.daily)[31879]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 11 06:09:09 myserver run-parts(/etc/cron.daily)[32519]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 12 07:40:40 myserver run-parts(/etc/cron.daily)[21853]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 12 07:40:41 myserver run-parts(/etc/cron.daily)[17894]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 13 09:23:28 myserver run-parts(/etc/cron.daily)[6385]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 13 09:23:29 myserver run-parts(/etc/cron.daily)[8172]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 14 07:42:36 myserver run-parts(/etc/cron.daily)[28652]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 14 07:42:37 myserver run-parts(/etc/cron.daily)[26861]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 15 08:42:19 myserver run-parts(/etc/cron.daily)[7882]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 15 08:42:20 myserver run-parts(/etc/cron.daily)[24861]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 16 06:12:51 myserver run-parts(/etc/cron.daily)[32686]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 16 06:12:52 myserver run-parts(/etc/cron.daily)[19153]: finished nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 17 04:20:27 myserver run-parts(/etc/cron.daily)[25207]: starting nethserver-check-nsreleaselock
/var/log/cron-20181118:Nov 17 04:20:28 myserver run-parts(/etc/cron.daily)[4571]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 18 04:05:10 myserver run-parts(/etc/cron.daily)[29499]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 18 04:05:11 myserver run-parts(/etc/cron.daily)[11436]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 19 03:13:35 myserver run-parts(/etc/cron.daily)[15959]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 19 03:13:36 myserver run-parts(/etc/cron.daily)[19729]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 20 08:25:17 myserver run-parts(/etc/cron.daily)[25880]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 20 08:25:18 myserver run-parts(/etc/cron.daily)[13074]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 21 07:43:38 myserver run-parts(/etc/cron.daily)[19634]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 21 07:43:39 myserver run-parts(/etc/cron.daily)[13731]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 22 06:24:47 myserver run-parts(/etc/cron.daily)[4023]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 22 06:24:48 myserver run-parts(/etc/cron.daily)[20418]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 23 07:59:21 myserver run-parts(/etc/cron.daily)[31631]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 23 07:59:22 myserver run-parts(/etc/cron.daily)[26006]: finished nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 24 04:57:28 myserver run-parts(/etc/cron.daily)[20775]: starting nethserver-check-nsreleaselock
/var/log/cron-20181125:Nov 24 04:57:29 myserver run-parts(/etc/cron.daily)[6660]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 25 04:51:27 myserver run-parts(/etc/cron.daily)[8128]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 25 04:51:28 myserver run-parts(/etc/cron.daily)[19008]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 26 03:35:14 myserver run-parts(/etc/cron.daily)[7429]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 26 03:35:15 myserver run-parts(/etc/cron.daily)[10886]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 27 09:31:00 myserver run-parts(/etc/cron.daily)[2095]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 27 09:31:01 myserver run-parts(/etc/cron.daily)[13560]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 28 03:35:37 myserver run-parts(/etc/cron.daily)[15772]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 28 03:35:39 myserver run-parts(/etc/cron.daily)[5903]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 29 08:25:09 myserver run-parts(/etc/cron.daily)[8882]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 29 08:25:10 myserver run-parts(/etc/cron.daily)[16572]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 30 05:18:52 myserver run-parts(/etc/cron.daily)[9027]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Nov 30 05:18:53 myserver run-parts(/etc/cron.daily)[5034]: finished nethserver-check-nsreleaselock
/var/log/cron-20181202:Dec 1 03:34:12 myserver run-parts(/etc/cron.daily)[8279]: starting nethserver-check-nsreleaselock
/var/log/cron-20181202:Dec 1 03:34:12 myserver run-parts(/etc/cron.daily)[28579]: finished nethserver-check-nsreleaselock
Yes, that seems to be the problem.