site builder, checkout only. renewed domain with CA, retrieved SSL certs (standalone, then with CSR file). Domain page in ns (change certificate, manual) refuses to accept any of the above, claiming a lack of intermediate certificate. Tried uploading both at once, same issue. Not taking either set of certs. Thoughts?

See Suiteanswer 27466 to check compatibility. Try a certificate from Let's Encrypt if you're not already doing so. If you still struggle, it probably needs raising with support.