The problem with this setup is, that you have only one domain.
It is not possible that the nameserver of a domain (petiteboitesansfond.net) is only reachable by the domain itself or a subdomain (ns1.petiteboitesansfond.net).
Thats the Hen <==> Egg problem
If you have only one domain, I recommend that you create the A and MX records for petiteboitesansfond.net in the DNS servers of your registrar ovh.net and do not use your local DNS manager in ISPConfig. If you get a second domain, you can configure the second domain in ISPConfig DNS-Manager without problems.