Custom Workflow error when converting to Simplified Workflow

Current Behaviour

When attempting to convert a simplified workflow the error "Workflow migration did not complete successfully." is generated, as in the attached screenshot.

Expected Behaviour

The workflow is created successfully, or a more description error message of why it failed is generated.

Workaround

1. Go to the Workflow Schemes in Project Administration.
2. There will be a Workflow Scheme named "GreenHopper Simplified Workflow Scheme for Project X".
3. Modify the project so that it uses that scheme.