Denny,
Looks like he made a mistake with the WordPress Address (URL) and Site Address (URL).
Almost all of the URLs are lacking the /cms/ needed to get your site to work.
So this does not work:
http://www.vlielandvantoen.nl/wp-content/themes/weaver/readme.txt
But this does:
http://www.vlielandvantoen.nl/cms/wp-content/themes/weaver/readme.txt
Try this method
http://codex.wordpress.org/Changing_The_Site_URL#Edit_functions.php
Download via FTP a copy of /cms/wp-content/themes/weaver/functions.php and save that somewhere safe.
Modify a copy of that file so that the end of that file looks like this.
add_action('wp_head', 'weaver_wp_head');
add_action('admin_menu', 'weaver_add_admin');
add_action('admin_head', 'weaver_admin_head');
update_option('siteurl','http://www.vlielandvantoen.nl/cms');
update_option('home','http://www.vlielandvantoen.nl');
?>
The last two update_option lines are new.
Now upload the copy and overwrite the existing file on your server /cms/wp-content/themes/weaver/functions.php
Visit your blog and see if that fixes it. If it doesn't try replacing the two update_option lines with these.
update_option('siteurl','http://www.vlielandvantoen.nl');
update_option('home','http://www.vlielandvantoen.nl/cms');
I often get those two option URLs mixed up when they're not the same... ;)
Once you are back to normal, put back that original functions.php file.