Same problem for me… Manual upgrade (with the procedure), and got this same error code for all pages (blog and admin pages). Downgrading to 2.7.1 waiting for a fix.
nice trick baardvidar,
You can replace
$wp .= ',' . 'http://your_theme_url/wysiwyg.css';
by
$wp .= ',' . get_bloginfo('template_url') . '/wysiwyg.css';
