The settings in ispconfig are ok. Please check the file permissions of all files in the website (all files in the "web" directory, they must be owned by the web user and client group of the website.
If you checked or changed that and it still does not work, take a look at the error.log of the website and post the messages that you get there.
suphp is not a good choice, its much slower then fastcgi + suexec and tends to make problems with larger cms. The important part is that you use suexec + fastcgi as this enabled apache to run the php process under the correct user similar to suphp while using fastcgi without suexec would run php under a wrong user.