Error when launching workflow service

Hi,

I am working through the Contract Approvals tutorial app. I got to the point of testing the app and when I got to the step where the workflow is intitiated - an error occurs. Looking at the error it doesnt provide much details so I was wondering if someone could help point me in the right direction. Thanks :)

Error -

{"message":"Internal server error","exception":"Invalid URL exception occurred"}

Details -

1. Request URL: https://na-1-dev.api.opentext.com/workflow/v1/process-instances
2. Request Method: POST
3. Status Code: 500 Internal Server Error
4. Payload: {"processDefinitionKey":"contract_approval","name":"Approve contract","outcome":"none","variables":[{"name":"base_url","value":"https://na-1-dev.api.opentext.com"},{"name":"contract_id","value":"eb13de39-d8b3-48d6-b3c4-64c2cb334bf0"},{"name":"line_manager_approval_acl_id","value":"f2c6f733-a7ab-403a-8346-035f3c541b47"},{"name":"risk_manager_approval_acl_id","value":"44503ffa-4b32-405b-833a-50132cfe8a8b"},{"name":"completed_acl_id","value":"4ef320b2-37cd-4bc0-88e1-bfbca9ade0c4"}],"returnVariables":true}