Oh, my mistake.. I forgot that it was a jailed httpd. There was a "File Not Found: /usr/lib/apache/modules/mod_throttle.so" message in the error_log, but the file was already there (out of chroot path). So, I copied the mod_throttle.so file into /var/www/conf/modules and changed the path of LoadModule throttle_module in httpd.conf.
'apachectl restart' is working again. Thanks. // Ozgur
