Why Does a Supplier See an Invalid Certificate Error on My Website?

The website certificate appears to be valid and is issued by Let’s Encrypt, so the problem is probably on the supplier’s side rather than with the site itself. Their computer may have an outdated certificate store, may not trust Let’s Encrypt’s ISRG Root X1 certificate, or their company network could be inspecting and replacing HTTPS certificates. Have them test the site from a different computer and a separate internet connection, such as a phone hotspot. Their IT department may also need to update the operating system or browser certificate store.

That error usually means the browser cannot build a trusted chain from your site’s certificate to a recognized certificate authority. Possible causes include an old operating system, restricted corporate certificate policies, or a server configuration that is not sending the required intermediate certificate. Since your host manages the certificate and other people can access the site normally, ask Zoho to verify that the complete certificate chain is being served and have the supplier’s IT staff check their trusted root certificates.

If both browsers fail on the same company computer or network, switching browsers probably will not help. The supplier should try the site from another device and network, then ask their IT department to check for HTTPS filtering or certificate interception. They can also compare the certificate details with an independent SSL testing service. Avoid installing a new certificate blindly unless the hosting provider confirms that the certificate chain itself is misconfigured.