Try the following:
- Enable debugging (example here). If you can't get to the page in Moodle, add the lines to config.php (example here).
- Edit line 27 of theme/styles.php commenting out the line define('NO_DEBUG_DISPLAY', true); by putting // at the start of the line.
- Edit line 28 of theme/yui_combo.php and again comment out the line define('NO_DEBUG_DISPLAY', true);.
Hopefully then some error information will be added to the error log on the web server, or maybe sent to the browser.