Got the following Error (Debian 11) when following the intructions from "securing-ispconfig-3-with-a-free-lets-encrypt-ssl-certificate" /usr/local/ispconfig/interface/ssl# cat ispserver.{key,crt} > ispserver.pem cat: ispserver.key: No such file or directory cat: ispserver.crt: No such file or directory It seems that symlinks are handled different now in Debian 11.
All current ISPConfig versions create the LE certs on their own at install time and you can also easily recreate them using ISPConfig updater. The tutorial you used (as mentioned in the tutorial explicitly at the top) is not compatible with any ISPConfig release that has been released in the past few years. Why do you want to destroy your setup by following this guide that explicitly mentions that you can not use it on your system?
Thank you, Till! Your answer made me rethink my intentions. I have deleted the browser cache and it works now. Btw, it's just a test setup. I've used to work with ISPconfig intensively in the late 2000's and I'm getting to use it again now, because I'm a huge fan still!!