After 30 September 2021 you can see the "License expired" message on your website and when you try to set up the license key on the WPFTS Settings / Licensing page, you may notice the red error message saying "Update API is not accessible".
The reason for this behavior is the expiration of the "DST Root CA X3" root certificate (expired 30 Sep 2021) that is required to connect to all Let's Encrypt-based services and the WPFTS Update service in particular.
How you can solve this?
In case your server is a shared service, you need to contact your hosting provider and ask to renew the root certificate bundle.
In case you have a VPS or Dedicated machine, you need to update root certificates by yourself.
On the Ubuntu/Debian machines, just disable the certificate on your server. Run:
sudo dpkg-reconfigure ca-certificates
On the first screen that prompts "Trust new certificates from certificate authorities?" choose "yes". On the next screen, press the down arrow key on your keyboard until you find
mozilla/DST_Root_CA_X3.crt, press the space bar to deselect it (the [*] should turn into [ ]), and press Enter.
Also please update and upgrade installed packages using
sudo apt-get update
sudo apt-get upgrade
For the CentOS machines:
yum reinstall ca-certificates
These actions should fix problems with the SSL certificates.