30th septembre 2021 is the expiration date of a root certificate used by Let'sEncrypt. The following page explains how it may impact developers:

https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

As a WINDEV / WEBDEV / WINDEV Mobile developer, you don't have to worry if you're using version 25, 26 or greater. Only users still using a version lower than version 25 will be impacted.

There are two solutions:

1. Easiest and obvious one is to upgrade your application to version 25 at least.

2. Ask the server admin to build a new certificate for the site such as the "ISRG Root X1" root certificate is the preferred one. 

Explanation: Let's Encrypt is using two root certificates, the (DST Root CA X3) and the (ISRG Root X1). Applications built with version 24 or lower are only using the (DST Root CA X3) because it was the preferred one.
When building the certificate, there is a way to use a --preferred-chain "ISRG Root X1" parameter than will allow version 24 apps to use the right certificate.