The process works fine, I have tested it. When I click on "Check Challenge", this is what I get:

Challenge not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_EXECUTE_FLOW_TRIGGER, The record couldn’t be saved because it failed to trigger a flow. 
A flow trigger failed to execute the flow with version ID 3012800000001r1. 
Contact your administrator for help.: []