I've installed letsencypt and done the following to symlink to the files: Code: # Change into the directory of your sites SSL files. # You will need to update the site name to that of the site created in ISPconfig. cd /var/www/sites.xxx.com/ssl # Remove all existing files. # you should take a backup of these files before this command. rm -r ./* # Now create the symlink. # You will need to update the site domain for this to work for your new site. ln -s /etc/letsencrypt/live/sites.xxx.com/cert.pem sites.xxx.com.crt ln -s /etc/letsencrypt/live/sites.xxx.com/privkey.pem sites.xxx.com.key ln -s /etc/letsencrypt/live/sites.xxx.com/chain.pem sites.xxx.com.bundle But in firefox I get the following error: xxx.xxx uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. Has anyone come across this before and know how to resolve it?