Just to make sure, you used the Plesk DNSSEC extension to generate the DNSSEC records on the domain then logged into the registrar (where you registered the domain) to add the generated records? Because the records you get from the extension is meant to be inserted where you've registered the domain on, not where you manage the DNS records. So if the DNS records is managed by OVH (and not on your Plesk server) and your registrar is somewhere else, then you would need to make the DNSSEC record from OVH to install with the registrar. If the DNS is actually managed by your Plesk server then you will want to make sure the that the name servers on the registrar actually points to your server and the DNSSEC entries generated and install the digest info into whoever the registrar is.