Workflow task "Invalid source in Assignment"

I'm trying to assign a second task to a user upon completion of the first task using the Assign To tab within the task. The user is assigned the second task via a Workflow field which I've called "Employee Name" and the data type is set to "User of a Group" with a predefined entry from the index entry of the document which starts off the workflow. The document is a form and I've double checked that the Employee Name field in the form is connected to the Filing Cabinet via the Indexing tab. In the Workflow Manager I am getting an error which says "Invalid source in assignment 1" "Invalid source in assignment 2" etc all the way up to five.

Does anyone know what may have caused this?

Thanks in advance

Dear Rhys Woolley! It looks like the Community cannot answer your question. That's why we have opened a Support Request with the Number SR-152070-L8T4R for you. A Software Support Specialist will contact you directly to follow up. We will update this thread with the solution as soon as we have resolved the Support Request. With best regards, DocuWare Support Team