Safepay Rejects Certificates From Gandi Sas

I'm trying to license some backup software, from this site:


https://www.bvckup2.com/purchase/


If you go there using Safepay you will see this message:


The security certificate for this page is not trusted.


Bitdefender Safepay™ attempted to connect to bvckup2.com


and the following error(s) were encountered:


The server's certificate is signed by an untrustworthy authority.


The server's certificate is not issued by a trusted certificate authority.


Upon checking the certificate I see it was verified by Gandi SAS. I've never heard of Gandi before, and have no opinion on their trustworthiness.


Is Gandi genuinely untrustworthy? Or is there some other problem? Why does Safepay reject them?