Dear Till I have read the posted articles but the solution doesn't seem to work.
You should not add the string 'AddType application/x-httpd-php .php .php3 .php4'. You should add the word 'addtype'. The options of the variable are described in config inc.php. To make it absolutely clear:
1) Open the file /home/admispconfig/ispconfig/lib/config.inc.php in a editor of your choice.
2) change the apache2_php variable, so that it looks EXACTLY like this:
$go_info["server"]["apache2_php"] = 'addtype';
3) Save the file.
4) Go to the ISPConfig interface, cahnge a website and click on the save button.