nav-menus.php returns 404 not found

I can not access nav-menus.php, it return a 404 not found page. I have installed a fresh wordpress with nothing posted and plugins, I just add categories and menu items, it become not working after I add certain amount of menu item.

For my real site, I tried the following: I deleted .htaccess file, the nav-menu.php return an internal server error page, but all other files in the back admin area are all working. .htaccess file is required for using custom permalink. After I put .htaccess file back, it return a 404 not found. I can no longer access the menu panel in the admin area.

my php version 5.2, memory set to 128 Mb. Now I aming using a menu plugin to do the menu.