Luckily I log all PHP errors and the error log told me where the error was (an old plugin I've forgotten to remove) .. phew

To log errors, I have the following statement at the top of my wp-config.php:

@ini_set('log_errors','On');<br />
@ini_set('display_errors','Off');<br />
@ini_set('error_log','/PATH/log/php_errors.'.date('Y-m-d').'.log');