X_MUTATION_PATH_UNFILLED
Means
a pinned action's :param had no value in the write's input
Typical cause
useMutation or the outbox replay was handed an input with no usable value for a :param of the action's http.path pin, so no URL could be filled — the write was not sent
Fix
pass the param in the mutation's input: mutate({ <param>: '<value>', ... }), the key the pin's :<param> names