OK. I made another (little) step.
Phpmyadmin or webmail is working fine only in mod-php mode.
When I switch to Fast-CGI, CGI, Su-PHP, or PHP-FPM the appear.
So, what should I do? What's the problem?
Can You Help Me?
PS: At the time of installation I chose to change the ISPConfig listen port setting from 8081 (default) to 9091 (my custom port) for instance. Can this be the cause of my problems?
I ask that because in /etc/apache2/sites-available/apps.vhost the port remain 8081 and changing to my new port 9091 apache failed to start.