HowTo create a new phpBB forum when running multiple forums with a single installation of phpBB (see: multiple phpBB-forums)
Note: For database manipulation, I'm using phpMyAdmin.
1) create new database
2) create new db-user:
give it NO global rights at all and
only localhost-access
3) grant new db-user full access to new forum-db.
4) Add entry for new DB to "phpBB/config.php"
5) Add apache-alias from new forum-url to phpBB.
6) move folders "install" and "contrib" back to their original places. Apply correct access rights: 755.
7) open the new forum's URL in browser with subdir: "install/install.php"
8) enter data.
9) remove "install" and "contrib" folders
10) voila.