Backport issue 426 / PR 429 to master

Yaml config files into `app/config` are not valid

This issue has already been reported one year ago with issue #426, and has been fixed thanks to PR #429
However, it seems that this PR has been merged into branch 2.1, and not backported into master

Therefore this bug still exists in master, 2.2, 2.3 and 2.4

Here is a PR to merge into master (and it would need a backport into 2.3 and 2.4 at least ?, and maybe a new tag for each)

The fact is I need to parse those yaml files with a Node.js module, and it just throws an error, because of this invalidity