apache2.conf erroneously includes vhost configs twice, and therefore apache2 service will not work.
Apache2 service failure after update from ISPConfig 3.2.7 to 3.2.8p1 - symptoms were that the service did not start after ispconfig_update.sh with reconfigure-services yes (the default) This occurred on a server which is ubuntu 18.04 but did not occur on a server which was ubuntu 20.04.
systemd[1]: Starting The Apache HTTP Server... apachectl[3195]: AH00526: Syntax error on line 6 of /etc/apache2/sites-enabled/000-apps.vhost: apachectl[3195]: Cannot define multiple Listeners on the same IP:port apachectl[3195]: Action 'start' failed. apachectl[3195]: The Apache error log may have more information. systemd[1]: apache2.service: Control process exited, code=exited status=1 systemd[1]: apache2.service: Failed with result 'exit-code'. systemd[1]: Failed to start The Apache HTTP Server.
apachectl --t -D DUMP_VHOSTS AH00526: Syntax error on line 76 of /etc/apache2/sites-enabled/100-domain.com.vhost: FastCgiExternalServer: redefinition of previously defined class "/var/www/clients/clientX/webY/cgi-bin/php-fcgi-IP-80-domain.com" Action '--t -D DUMP_VHOSTS' failed. The Apache error log may have more information.
Server: Ubuntu 18.04.6 LTS (Bionic Beaver)) ISPConfig 3.2.8p1
FIX: /etc/apache2/apache2.conf - comment out one of the below lines, as they're including the web config files twice.
Include the virtual host configurations:
IncludeOptional sites-enabled/ IncludeOptional sites-enabled/*.vhost