We observe regular "impossible happened" error on main zabbix server:

[[email protected] /]# zgrep -iEc 'lld_item.c,line:1958.+just happened' /var/log/zabbix/zabbix_server.log*|grep -iv :0
/var/log/zabbix/zabbix_server.log:667
/var/log/zabbix/zabbix_server.log-20171002.gz:4376
/var/log/zabbix/zabbix_server.log-20171008.gz:773
/var/log/zabbix/zabbix_server.log-20171016.gz:4867
/var/log/zabbix/zabbix_server.log-20171029.gz:12902
/var/log/zabbix/zabbix_server.log-20171105.gz:976
/var/log/zabbix/zabbix_server.log-20171113.gz:4748
/var/log/zabbix/zabbix_server.log-20171126.gz:3134
/var/log/zabbix/zabbix_server.log-20171203.gz:2035
/var/log/zabbix/zabbix_server.log-20171211.gz:2553

reasons is not known. Nothing interesting in log around this messages (daemon starting/stopping only).