Admin area problem

Hi there!! This afternoon I had 3 times a server 500 error. Cause unknown, but afterwards the CSS of the admin area does not load anymore. I searches for solutions, but nothing I found and tried worked. I re-uploaded WP and the plugins directory, also no result. Debugging shows:

Notice: wp_enqueue_script was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or login_enqueue_scripts hooks. Please see Debugging in WordPress for more information. (This message was added in version 3.3.0.) in /var/www/vhosts/relaxmore.nl/httpdocs/wp-includes/functions.php on line 4148

Notice: Constant FS_METHOD already defined in /var/www/vhosts/relaxmore.nl/httpdocs/wp-config.php on line 51

What can I do with this?
Your help is appreciated!
Cheers,
Ronald