I can’t get to pages in the Admin that use Dir/file.php

You running straight 1.5 Strayhorn release?

And when you say “it just shows the regular WP Admin interface but no content”, what does that mean? Do you mean it has the appropriate headers — including perhaps even a highlighted “tab” for the current page — and then the footer immediately following?

Has this always happened or did it just start?

Waitaminute… are you on a Windows or Unix or Apple host? Didn’t I recall you having trouble with pathnames before? Or am I thinking of someone different?

Save a copy of your wp-admin/admin-functions.php file and edit it. Replace the add_menu_page() function with this version and see if it helps:

function add_menu_page($page_title, $menu_title, $access_level, $file, $function = '') {
global $menu, $admin_page_hooks;
$file = plugin_basename($file);
$menu[] = array($menu_title, $access_level, $file, $page_title);
$admin_page_hooks[$file] = sanitize_title($menu_title);
$hookname = get_plugin_page_hookname($file, '');
if ( !empty($function) && !empty($hookname) )
add_action($hookname, $function);
return $hookname;
}

In what way? It does the same thing?

I’m at a loss. If you know PHP this may be the time to start narrowing it down with echo calls to print out important stuff in the admin.php file. Figure out which path it’s heading down in the part that says

// Handle plugin admin pages.
if (isset($_GET['page'])) {

by echoing the $page_hook variable and such. Goofy indeed.