Backport issue 426 / PR 429 to branch 2.2

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 branch 2.2 (and it would need a backport into master, 2.3 and 2.4 ?, and maybe a new tag for each)