Zabbix Agent 2 5.0 - all plugin based stuff is inside the config file.
Zabbix Agent 2 6.0 - plugin configuration can be put in separate files in conf.d/ folder using include option
In 6.0 the default parameter for this is set to this relative path:
Under Include parameter definition, there are only absolute paths specified as valid ones:
The problem here is if you use the agent configuration file with Zabbix Sender.
You run sender command from lets say home directory like this:
And then you will get this error:
This happens because zabbix_sender does not know this relative path.
I would propose moving the relative directory to an absolute one: