That seems to be a mistake in the documentation. The class responsible for handling that call only returns the png, see https://github.com/flowable/flowable-engine/blob/master/modules/flowable-rest/src/main/java/org/flowable/rest/service/api/runtime/process/ProcessInstanceDiagramResource.java#L77
The body shown in the docs there, seem to be for /process-instances/{id}. I’ve removed the wrong body from the docs: https://github.com/flowable/flowable-engine/commit/2d20598edb24a2e6494927187494a001fa690839
The call you’re looking for is this one: https://flowable.org/docs/userguide/index.html#_get_active_activities_in_an_execution