Hey Auge, thanks for your help. I keep on writing in English, hoping more people will be able to help me with this.

I just tried again. This is exactly what I did:

1. downloaded my little forum again, unzipped it
2. uploaded all files to /forum using Filezilla
3. set all folders/subfolders to 755
4. set all files to 644
5. set templates_c to 775
6. set db_settings.php in config to 777
7. opened /forum/install
8. entered all required information
9. started the script
10. got these error-messages:

Fehler!

* SQL-Fehler (MySQL: Table 'mlf2_banlists' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_categories' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_entries' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_settings' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_smilies' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_userdata' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_pages' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_useronline' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_logincontrol' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_entries_cache' already exists)
* SQL-Fehler (MySQL: Table 'mlf2_userdata_cache' already exists)
* SQL-Fehler (MySQL: Duplicate entry '1' for key 1)
* SQL-Fehler (MySQL: Duplicate entry '2' for key 1)
* SQL-Fehler (MySQL: Duplicate entry '3' for key 1)
* SQL-Fehler (MySQL: Duplicate entry '4' for key 1)
* SQL-Fehler (MySQL: Duplicate entry '5' for key 1)
* SQL-Fehler (MySQL: Duplicate entry '6' for key 1)
* SQL-Fehler (MySQL: Duplicate entry '1' for key 1)

11. opened Plesk Panel 9.2 in new tab
12. deleted mysql-database
13. created new database
14. went back to old tab, entered new mysql-information and started the script again
15. ended up seeing a blank page again...

Now it doesn't matter if I go to /forum/install or just to /forum, all I get is a blank page. The script seems to be doing something, but I guess it never gets to the end.

Thanx again!