I too am having the same issue. My process is also very simple, and I can't seem to find where I could have made a mistake.
I googled the error and found only one or two possible solutions that involved changing my data source. I don't think that is exactly the solution to either of our problems. It seems a bit complex for such simple workflows.
Yes you are right. I hope i could get proper documentation on all the small small things we need to consider . It is very simple process and still my form is not getting submitted
Make sure you are not checking the Submit Service checkbox in the xfaForm variable in your workflow.