There's nothing in the log file that is useful. I also checked the .htaccess file and restored it to the initial state.
I think the issue might be the fact that the symlink name is different from the application name. I couldn't create a symlink with the same name because it didn't let me. So if the name of the application is "app" then the symlink is "apps". If I just type in web.com/app, it would not direct to the public directory, but gives me a list of directory instead.
Any suggestion? BTW, this is an application in a subdomain, not subdirectory.