Do you have any url rewriting going on? I notice that a request for sheet=additional.css is returning "/* Unable to find css sheet */", which, looking at the code in css.php, should only happen if there is no "sheet=" specified on the url.
ETA: oh, I just noticed you have that message in the title.
Anyway, another possibility is that a plugin is messing thing up - did you try disabling all plugins (put define('DISABLE_HOOKS', true); at the top of config.php, under the <?php line)?