Well, if you are not firing any plugin at service entity create event then
you need check followings:
1) Restart your Async Service and try to execute your workflow.
2) Make sure you are not firing any plugins at service entity events, if yes
then check for plugin errors.
3) Enable CRM Server Trace and look into Async service Trace File. This will
exactly show you the error and its detail.
Hope this helps. If not do let me know!
Ayaz Ahmad [MVP - MSCRM]
Technical Consultant CRM & SharePoint