Thanks, KayNine, but I fixed the error.
My WP install was not in the root directory, as it assumed, so the required file could be found in /wordpress/wp-admin/post.php. I only just had to change the address bar URL.
I guess this was because of what the WP setup did in the Step 2 of the install, where it tries to guess the WP location itself, but asks you neverthless if it is correct. I just went with the default, which was my server-root. Maybe that’s what the problem was.
Once I logged in, I changed the paths appropriately in the options, and everything’s fine. Thanks a heap.