Zabbix runs as part of Pacemaker cluster.
There were 2 reasons:
1. mySQL did not have enough connections allowed
Set this parameter in my.cnf: max_connections = 512
2. Zabbix service controlled by Pacemaker. This requires maintenance mode. I saw articles on how to do it with crm command but this is obsolete. So here is how with pcs
pcs property set maintenance-mode=true
pcs property set maintenance-mode=false
Once it is in maintenance mode, it can controlled independently of Pacemaker.