Play all jobs for a stage shows only an error

Summary

When clicking on the Play-all-button an error is shown.

Steps to reproduce

1. Go to a pipeline
2. Click on the "Play all manual"-button for any stage

What is the current bug behavior?

An error-layer is shown: An error occurred while making the request. The network request returns 400 (Bad request) : POST https://domain/repo/pipelines/1234/stages/stagename/play_manual.json

What is the expected correct behavior?

All jobs of a stage should be executed.

Relevant logs and/or screenshots

No entry in /var/log/gitlab//.log

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:
Current User:   git
Using RVM:      no
Ruby Version:   2.6.3p62
Gem Version:    2.7.9
Bundler Version:1.17.3
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.21.0
Sidekiq Version:5.2.7
Go Version:     unknown