Backend: Pipeline does not run or shows an error when scheduled

Summary

It is not possible to schedule a specific pipeline if it contains except:schedules and only:schedules jobs and some dependencies.

Steps to reproduce

I reduced my problem down to the following pipeline

A:
  except:
    - schedules
  script:
    - echo hello

B:
  needs:
    - A
  # this does not help
  # except:
  #   - schedules
  script:
    - echo hello

C:
  # never scheduled
  only:
    - schedules
  script:
    - echo hello

Example Project

https://gitlab.com/fhoffmann_vibim/gitlab-pipeline-schedule-test

What is the current bug behavior?

Scheduling the pipeline to a specific time or clicking the play button on the schedule page does not create a new scheduled pipeline or error message.

What is the expected correct behavior?

The job C should be executed if the pipeline is run by the scheduler.

Gitlab should report correct error messages if there is an issue with this pipeline.

It should also be possible to exclude all jobs from the pipeline except one job when the the scheduler runs the pipeline.

Relevant logs and/or screenshots

Gitlab produced no error message.

Results of GitLab environment info

The problem was repoduced on gitlab.com