1) Download the up-to-date root certificate (ISRG Root X1) from https://letsencrypt.org/certs/isrgrootx1.der 2) Using the KeyChain Access app, import the certificate into your 'login' or 'system' chain 3) Find the certificate ISRG Root X1 in the chain where you placed it, double click on it, open the "Trust" area and set "When using this certificate" to "Always Trust" (you will be asked for your login password when doing this). I have done this on an old iMac running El Capitan, and it solved the problem. For more info / confirmation of this approach by Old Unix Geek, see https://mjtsai.com/blog/2021/09/24/some-web-sites-will-stop-working-with-el-capitan-and-older/#comment-3538503 Ted
