This sounds like My issue, First the reason you are getting this error is because during the install the prepend.php is created in  dotclear\admin\install .

You will see this indicated in your Apache error.log file, you can even pull up phpmyadmin and see that the tables are created and have some data in the tables.   
So basically dotclear is installed .
Did you try to login at http://<Your Server Name>/dotclear/admin/auth.php