Document Generation step in workflow

Hello,

I added a “Document Generation” step to my workflow. While testing I can see, that the step is executed without errors. However, when checking workflow activity I see that my output variable for generated configuration is empty although I supply an input variable with exactly the same XML schema and some data in it. DocGen form fields are set as not mandatory - “Require user input” is removed. 

What else should I check in my workflow? Which XML schema should I apply to the input variable for the “Document Generation” step?

Thank you