Hi all, if it helps, this article https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/ talks about the root CA X3 expiration.
With all Browsers, it worked fine but with (java) tools working on the url, I had the issue of expired part.
I'm maybe not using the same way as you all, because I'm using le64.exe to generate my certificates but I have the same issue of the intermediate certificate expired.
To resolve the issue, I've removed the expired part from my certificate.
I've checked the complete chain with this site https://tools.keycdn.com/ssl and the url after the replacement with this site https://decoder.link/sslchecker/.
Everything looks good.
It's probably not the most automated solution but it solved my issue and my certificate works fine at 100% in all situations.
I'm not an expert but if you have any idea of how to generate the cert without the expired part at the end, I'm interested.