Hi,
The new version 5.5.5 of Certify The Web is available for Windows OS (Desktop and Server) from https://certifytheweb.com/.
This release includes a special new Certificate Authority maintenance feature which will update key certificates for known certificate authorities. In particular the version will automatically (on first run) ensure the ISRG Root X1 root is trusted, and ensure the correct R3 intermediate is present. This resolves common issues on windows servers where the Leaf > R3 > DST Root CA X3 chain is continuing to be served (when used with IIS and other http.sys enabled servers).
Note that even if you use an alternative ACME client (such as win-acme, Posh-ACME etc), installing this update may help resolve common trust issues automatically, even if you don't then use the app or just uninstall it later. Some users may find that a reboot is required after installing in order to get their server to use the updated ISRG Root X1 chain. Certbot users are not affected unless certificates are being converted to PFX and stored (apache, nginx etc don't use the windows trust store for chain building).
More information regarding Certify The Web and the Sept 2021 Let's Encrypt root expiry can be found here: Let's Encrypt DST Root CA X3 expiry Sept 30th 2021 | Certify The Web Docs