In my scenarion my process should receive a signed PDF which I should show to the workspace user.
I did this by selecting the document input variable as the input for the "Assign Task-service". To be able to have the approve/deny buttons in workspace I added the process fields (don't know if they are neccesary) and set the submit
You can also try not adding the Process Fields, but instead, just add a submit button. A submit button is just a button set with a context type of Submit and also add the Form Bridge object to your form. Adding the Form Bridge object prevents the render service from augmenting your form, which invalidates digital signatures.
For more information about the adding a submit button to your form, see the Signature service > Best Practices > Requirements for form design and Workspace ES in the LiveCycle ES Services guide (http://www.adobe.com/go/learn_lc_services_82).