I used this tutorial to setup my ISPCONFIG: https://www.howtoforge.com/tutorial/ispconfig-automated-install-script/ Now everythings works fine except 1 thing! Then edit the file /var/lib/roundcube/plugins/ispconfig3_account/config/config.inc.php and set the password you inserted in ispconfig. See the following example: I cant find the path. ispconfig3_account is not their.
Did that script install ISPConfig Roundcube Plugin? Was there error during script or any message about plugin? I don't know what operating system you have. This tutorial may apply if you want to install and configure the Roundcube Plugin. https://www.howtoforge.com/tutorial/ispconfig-3-roundcube-plugins-on-debian-stretch/
The directory /var/lib/roundcube is there and contains files and folders, so RoundCube itself is installed? If yes, then the guide that @Taleman posted should probably work to install the ispconfig RoundCube plugin.
