Hi,
first of all: Your Icinga version is very outdated. It is over three years old.
So I suggest you update asap and use the official package repository for that.
https://icinga.com/docs/icinga-2/latest/doc/02-installation/#debianubunturaspbian-repositories
This is your problem:
Object 'registry.int.mydomain.tld' of type 'Zone' re-defined: in /var/lib/icinga2/api/zones/registry.int.mydomain.tld/_etc/registry.int.mydomain.tld.conf
Means that the configuration that the agent receives from the master is re-defining what the agent already knows, because it has the configuration for its own host/endpoint/zone object.
Did you do the setup on the agent with the node wizard?
https://icinga.com/docs/icinga-2/latest/doc/06-distributed-monitoring/#agentsatellite-setup-on-linux
Normally that should be enough for an agent and then add the host object with the exact same name as the endpoint, IIRC.
The troubleshooting link works for me.
Lastly: In future please use markdown to format your posts, especially when they contain config or logs:
This way the posts can be read more fluently 