Theme editing not allowed

I had a year ago problems with updating to a newer version of WordPress (from 2.8.6 to 3.0.1) on a test installation. Whatever I tried (disabling plugins, standard theme, checking database) I kept on getting the error

Sorry, can’t edit files with “..” in the name. If you are trying to edit a file in your WordPress home directory, you can just type the name of the file in.

Since my real blog is still running 2.8.6 I want to update it. So I gave it a new try with an update. Unfortunately with no success, the error just came back. I also couldn’t login due to another error:

Warning: main(./wp-load.php) [function.main]: failed to open stream: No such file or directory in ./admin.php on line 30

I don’t know if there is a relationship between the two errors, but this is how I solved it.
in admin.php (included in the directory wp-admin) I added a . before /wp-load.php

That solved the login problem

In file.php (included in wp-admin/include) I commented out the die statement in the validate_file function so it looks like this:

function validate_file_to_edit( $file, $allowed_files = ” ) {
$code = validate_file( $file, $allowed_files );

if (!$code )
return $file;

switch ( $code ) {
//case 1 :
// wp_die( __(‘Sorry, can’t edit files with “..” in the name. If you are trying to edit a file in your WordPress home directory, you can just type the name of the file in.’ ));

//case 2 :
// wp_die( __(‘Sorry, can’t call files with their real path.’ ));

case 3 :
wp_die( __(‘Sorry, that file cannot be edited.’ ));
}
}

I think this is a hard way, but at least I am able to work on the themes again with a newer WP version.

If anyone has a better solution (again, I have tried to disable the plugins etc), please let me know.