The description of the success response body for the endpoint:
GET runtime/process-instances/{processInstanceId}/diagram
Is not correct:
{
"id":"7",
"url":"http://localhost:8182/runtime/process-instances/7",
"businessKey":"myBusinessKey",
"suspended":false,
"processDefinitionUrl":"http://localhost:8182/repository/process-definitions/processOne%3A1%3A4",
"activityId":"processTask"
}
It actually returns a process diagram (blob):