When processing a form through a workflow, when it gets to an Assign Data or a Task's Assign Data step which writes back to the form via "Fill Area on the document" the following error is returned, 
Error in system activity: Cannot set PDF Form Fields to DocumentID: [] in FileCabinet: [] FIELD VALUES: [FieldName]: [Value]

This can be caused by any of the following, 

Scenario 1:
If this error is thrown from an Assign Data step, you will need to check the DocuWare account that is set to run the workflow.
Once this is known, you'll need to provide the "Edit documents" right on the file cabinet.

Scenario 2:
If this error is thrown from an Assign Data step inside a Task activity, then you'll need to determine which DocuWare account was assigned this task and tried to Confirm the decision for it.
Once this is known, you'll need to provide that DocuWare account "Edit documents" rights on the file cabinet.

Scenario 3:
If the previous scenarios do not apply, then you'll need to check which Windows account is set for the following, 

  • Platform Application Pool: This executes assignments that are done through the Task activity. Please ensure that this Windows account has access to the DocuWare system files, most importantly, the Storage location.
  • Background Process Server: This executes assignments which are done through the Assign Data step. Like above, please ensure that the Windows account has the appropriate permissions.

After making the changes depending on which scenario is relevant in your situation, the workflow should now be able to write back to the form.