We have a client who is receiving this error on one of our Pardot forms. The certificate doesn't show expiration until August. What does this error mean?
I've just tested, and you website is still secure and serving all required certificates (end entity + YR1 + YR by X1).
The error your showing (thank you for the screenshot) is from a firewall. The firewall might not be able to cope with three or more certificates in a chain (1 end entity and 2+ intermediates (in your case YR1 + YR)).
I'd suggest determining the firewall manufacturer and contacting them as this will become extremely important across the next two months.
Thank you! Why will this become more important over the next two months?
Thank you! Will our certificates auto renew?
Your certificates are fine. That error message you see doesn't look like something that's related to what your web server is sending, but something relating to the configuration of the network that the web browser is on. You'd need more details on what exactly is showing that error (possibly from the IT department of the network of the browser that's showing it) to know what the problem is exactly.
To phrase it another way: If a web browser also has trouble getting to https://valid.x1.test-certs.letsencrypt.org/ then they'll have trouble getting to any server that's using Let's Encrypt, and it's an issue with the browser and not with the server.