I have a brand new AEM OSGI instance on Windows. I upgraded to 6.5.13.
I have created a simple workflow with 1 step - No operation. No variables.
I have created a simple Adaptive form using the blank template. Added a text box and a Submit button.
I configured the form container to submit to an AEM Workflow and Previewed the form. It submits perfectly.
I created a Forms Application - I chose the Adaptive Form, The workflow, Assigned forms-users, and Relative to Payload - data.xml. Everything else is default.
I got to my Inbox (as admin), create application, choose my application. I see the form, I type some text and I click submit.
I receive an error in the UI, AEM-FD-011-003 - Error occurred while submitting draft.
No errors in log. I have not increased logging. I will try that now.
I had a quick look, found your form Submit Action set to Forms Portal Submit Action.
So I changed it to Invoke an AEM Workflow as you previously mentioned it should be.
The form submits.
I was able to reproduce your error from create application (inbox).
Try setting the "Store attachments using" of your MultiAssign workflow application (e.g. data-attach) an see if that gets you past your error.