WordPress won't load page, permalink is OK but $post is empty

Hello,

I have a strangest of all problems. A site that has a page structure with pretty permalinks enabled (/%category%/%postname%-%post_id%/) but when I change a page in a structure, this page is visible (in navigation) but when I click on it (from navigation menu) it displays 404 error page.

I tried:

1. removing all plugins – no success
2. removing any custom code from functions.php – no success
3. changing themes – sometimes ok, but generally no success (the same procedure once worked but other times doesn’t work)

When I navigate to this certain page (every page I change in structure (i.e. I make the page a subpage etc.) it just shows error. And I even tried to print_r($post) to see if there is something there, but the $post doesn’t exists – it’s empty.

Since I tried many things, I believe there’s something wrong with the WordPress itsself (if possible?).

Need your help ??