The operating system my web server runs on is (include version): Ubuntu 20.04 LTS
My hosting provider, if applicable, is:
I can login to a root shell on my machine (yes or no, or I don't know): Yes
I'm using a control panel to manage my site (no, or provide the name and version of the control panel): Yes, Plesk
Hi, I use Plesk to assign SSL Certificates but since the day DST Root CA X3 has expired, I am unable to download some packages that are stored on a repo that I have on this server. The composer is saying that "Certificate has expired" even though I have renewed this certificate.
From your description I guess the machine you run composer on is not validating the "long chain" you use for wprepo.k-clients.com
It may be missing ISRG Root X1 from the CA certificates store or it may just be objecting to the expired DST cert. I am not familiar enough with composer to know the verification technique it uses.
This is a good post about the short and long chains and the tradeoffs for each:
If that is not enough to resolve it for you. Please post back more details of your composer system (os version, ...) and other clients you need to serve (old Android, old openssl, ...)