Hi @brik,
The opensea.io site appears to be configured correctly according to Let's Encrypt's current recommendations.
The evidence is building up that some versions of macOS may have an unexpected incompatibility with a workaround that Let's Encrypt recommended for what would have been an incompatibility with older Android devices. If this is confirmed, it means that sites that choose the Android compatibility option will be incompatible with some macOS versions, while sites that choose the macOS compatibility option will be incompatible with some Android versions. 
Right now, it appears that this is something that would have to be fixed by the site operator, or by Apple, not by you. A temporary workaround might be to use Firefox to access these specific sites, because it uses its own trust store and certificate validation rather than those provided by the operating system.
The most detailed discussion of this issue on this forum is at
Be warned that some of that discussion is aimed at web site administrators rather than end users.