When installing ISPConfig 3 using the guide here:https://www.howtoforge.com/tutorial/perfect-server-ubuntu-16.04-with-apache-php-myqsl-pureftpd-bind-postfix-doveot-and-ispconfig/
After initial installation and configuration
When trying to access the webmail at https://YOURIP:8080/webmail/ you get a 404 error. If we select roundcube as our default webmail client
cat >> /etc/apache2/conf-available/local.conf <<EOF Alias /webmail /var/lib/roundcube EOF
a2enconf local
service apache2 restart
Don’t work
Were you using that specific version of Linux? If so than the issue may have been fixed and this code may no longer apply I did an ISPConfig 3 installation on Ubuntu 18.04 just yesterday using this guide https://www.howtoforge.com/tutorial/perfect-server-ubuntu-18.04-with-apache-php-myqsl-pureftpd-bind-postfix-doveot-and-ispconfig/ and it works flawlessly.