This error is most often seen because of nested runbook invocation from inline or function context. Such action is obviously not permitted. We can call Inline or Function from Workflow, but in other direction we have to use one of solutions described here.
I have just met another cause of such error.
In powershell, there is nomally available such syntax:
$var = (get-service)
It is using result of expression within brackets as a table, what is indexed without creation of intermediate variable.
Based on my expierience and some tests, I can state that this syntax is causing error “Root element is missing”