You need to login to your fantastico Admin panel and see it is properly or not.
AssistanZ - Beyond Boundaries... Cloudstack Consultancy / 24x7 Web Hosting Support / 24x7 Server Management / Infrastructure Management Services Web & Mobile Apps Development / Web Designing Services / Php, Grails, Java Development
I believe this has something to do with PHPSuExec being enabled or disabled on your server.
In the admin section of Fantastico there is a setting you can change to tell fantastico that you have or do not have phpsuexec. If this is set to the wrong setting, it will cause Internal Server errors with installed applications.