We recently had a couple of cases were in a Dynamics AX environment the Purchase Requisition Workflows would fail and the Workflow History showed the following status for these instances:
The affected AX environments had a security configuration that was relying heavily on separate user domains for the different AX user groups. None on these user groups except for the administrative users had any access to the Admin domain. Tests showed that the workflows were completing successfully if the submitting users had full access to the Admin domain.
In the end these issue could be resolved by selectively granting Open domain access permissions on the Admin domain to the wfsys user group.