my blog is OK, but admin pages are not

Have you checked your internal error logs?

Hi batrachoid, how can I check the log? I tried looking in the wordpress directory but found nothing regarding log files

Internal server errors (error 500) are usually described in more detail in the server error log. If you have access to your server error log (possibly available via your hosting provider’s control panel), generate the error again, note the date and time, then immediately check your server error log for anything useful. If you don’t have access to your server error log, ask your hosting provider to look for you.

Hello again,

We seem to have found the solution, but this is leading to something worse. Here is the error

Warning: require_once(./wp-load.php) [function.require-once]: failed to open stream: No such file or directory in ./admin.php on line 20
Fatal error: require_once() [function.require]: Failed opening required './wp-load.php' (include_path='.:/pear/lib:/var/www/pear/lib') in ./admin.php on line 20

To “fix” this we had to change line 20 in wordpress/wp-admin/admin.php from
require_once(dirname(dirname(__FILE__)) . '/wp-load.php');
to
require_once(dirname(dirname(__FILE__)) . './wp-load.php');
(add a dot before /wp-load.php).

The original line was telling the server to look for wp-load.php in the same directory as admin.php, when it was really one directory level above admin.php.
Why should we need to do that at all? Is this a (huge) WordPress bug or do we have some misconfiguration?

This is the first time that I’ve ever heard about that problem, so I’m going to lean on the side of a server misconfiguration, since we certainly would have heard about it far more often than just now.

Thanks James, do you have any clue on how to fix the configuration?

Not a clue, sorry.