“500 internal server error” when editting post/page in cloned site

i cloned a new site from my another exist site, copied&renamed all site tables, updated key data like userrole, siteurl and so on.

New site works, but cannot edit post/page (can edit category). Once i tried to edit a post or a page, it kept rolling and rolling in waiting response. By firebugs, i saw the server response is “500 Internal Server Error”, i searched a lot, verified file permissons, .htaccess file, disable all plugins, switch themes, but none is the real cause, any clues?

btw, new a site in normal steps will be no problem, i can export old site then import to new site, but wordpress import feature still has many bugs like data loss, duplicate data, so i have to do it by clone.