You can have the solution from this topic:
You could try restarting the FFCore system -- no guarantees.
I have a workflow that appears to have become corrupted and I cannot delete it. Initially, it said it was tied to a Hot Folder and couldn't be deleted, so I deleted the Hot Folder. Now, when I click on it in the Workflow List, it goes to a black screen with the words "Internal Service Error" and a red button that says "Close". Is there any way I can remove this from my Workflow list?