GitLab PipeLine Cannot be created (Undefined Error)

I am attempting to create my first (meaningful) child pipeline in GitLab.com and am suddenly coming across a meaningless error.

My parent pipeline has a job that create 5 identical child job files:

empty:
  script:
  - 'echo ''Intentionally empty pipeline (0)'' '
variables:
  node: 0

These files are then executed by 5 identical trigger jobs:

BuildSegment1:
  stage: compile
  trigger:
    include:
      - artifact: build/build.1.yml
        job: queuer

What's odd is that these trigger jobs fail at (seemingly) random. Sometimes 2 of them work, sometimes 3, somtimes 1. All failures give the same message:

downstream pipeline cannot be created, Undefined Error

I'm a little befuddled by where to even start looking to solve this.