Why Does My Website Show NET::ERR_CERT_AUTHORITY_INVALID for One Company?

If the supplier uses a corporate web filter or firewall, it could be intercepting HTTPS traffic or applying a policy to the newly registered domain. Their IT department should check the filtering logs and whitelist the domain if appropriate. Testing from outside their company network will help distinguish a site problem from an internal network issue.

It’s also worth having the hosting configuration checked to ensure the complete certificate chain is being sent, including the intermediate certificate and not just the certificate for the domain itself. A public TLS scanner can confirm whether the chain is complete and whether there are any server-side configuration problems.

The site’s certificate appears to be valid and is signed by Let’s Encrypt, so replacing it probably won’t help. The problem is likely on the supplier’s side: their computers may have an outdated trust store, or their company network may be inspecting or blocking the connection. Ask them to test the site from a different device and a different internet connection, such as a phone using mobile data.

That error usually means the browser cannot establish trust in the certificate issuer. Their systems may not recognize Let’s Encrypt’s ISRG Root X1 certificate, especially if the computers or browsers are old or heavily managed by their IT department. Updating the operating system and browser, or having IT update the trusted root certificates, may resolve it.