I faced this error Workflow stopped Error after submission for approval in Ax 2012.
My observation is as below
When I am create new Purchase order and purchase order is in invoiced status but 2 qty is in cancel mode then purchase order getting approved but for old order its give above error.
After investigation on google and other sites I did not get anything so finally I did approved workflow from back end activity.
I hope some people who is smart they will give answer to me on this query.
After some days I get same error again then I assigned admin rights to approval user then its getting approved.