If there's an error, you can fix the issue by recreating your initiation form and manually applying your source customizations to your destination form. In order to do this, you can perform the following steps :
- Open your destination site in SharePoint Designer.
- Open the Workflows section in the navigation menu on the left.
- Click on the workflow that is failing to start.
- Select your initiation form (usually named WFInitForm.aspx).
- Click on the Delete button in the Forms ribbon, as depicted in the following image:
- Save the workflow (this will recreate your initiation form without its customizations).
- Open your initiation form by clicking on its link.
- Click on Edit file:
- Manually reapply your form customization.
- Save your workflow.
- Validate that your workflow can now start correctly by launching it again in your browser.