thanks for your answer.
But this don´t work here, after changing the lines to
$system_user = 'www-data';
$system_group = 'www-data';
the fields in the database will not be updated.
Not for creating a new site, nor updating a site.
So I tried to manually change the fields in the database and after that changing details for the site in the ui to force an update. Now the user/group is www-data for the /var/www/mysite folder, but suddenly ISPConfig is not availiable anymore and i got an permission error? Only if I change the owner/group from ispconfig to www-data for all ispconfig scripts, the frontend comes back again.
Are there any other things to do? Does this work only by patching before ispconfig installation?
I´m using the latest ispconfig version under debian 6 with updated mysql and php (5.3.x) over dotdeb repository.