[SalesForce] Lightning Container refused to display in a frame because it set ‘X-Frame-Options’ to ‘sameorigin’ (2)

At the moment, we don't know the origin of the problem. We have set a new organization and this component works now.

HTML elements are rendered as aura:html components, so you need to get elelment first:

var iframe = component.find('iframe').getElement();

should return the html element, but be aware, that it will only be available after the component is rendered, as described here.

In case you are using a init handler I would suggest using render instead (docu):

<aura:handler name="render" value="{!this}" action="{!c.doStuff}"/>

So your action could call it like this:

iframe.getElement().postMessage({ test: 'test' }, '*');