Steps to reproduce:

1. Create an Trapper item via the API with an empty "allowed hosts" field (works with all zabbix server versions, except v3.4 ... a bug?) on v3.4 only '::/0' works fine ... as in the documentation described, "and '::/0' will allow any IPv4 or IPv6 address" "when IPv6 is enabled" (IPv6 is enabled and zabbix-server reports that on startup, too)
2. Send values to this trapper item

Result:

The Zabbix Server reports "xxx cannot process item "test3" trap: connection from "192.168.40.115" rejected, allowed hosts: '::/0'"

If we change the allowed hosts from '::/0' to '::/0, 0.0.0.0/0' it works

Expected:

• Zabbix Server accepts the values for trigger items when ::/0 is set, or the allowed hosts is empty 

We have over 90k Trapper Items in hundreds of software projects that reports values to our zabbix servers and with an update from 3.4 to 4.x the automatic creation of new items doesn't work anymore, but we can't upgrade all of our projects (the zabbix integration) for this.

Our Workaround right now will be a cronjob every minute to remove all '::/0' from new created trapper items (update items set trapper_hosts = '' where trapper_hosts = '::/0')