Hi Luc,
just tried it with the Web Client and it looks like you're right - the problem is with the self-signed certificate bundled with the OVA. This is an official NSX-T OVA that I downloaded from VMware. If I click "Proceed Anyway" then it's imported successfully.
It looks like there is no functionality to ignore OVA certificate with "New-ContentLibraryItem"? Perhaps this can be added in the future?
Thank you!