Another modifying header informaton problem

Warning: Cannot modify header information – headers already sent by (output started at /www/htdocs/xyz/wp-includes/wp-db.php:80) in /www/htdocs/xyz/wp-admin/post.php on line 711
This happend if i try to edit a comment from one Person. All the Comments from another Persons are not the problem.
I try to remove the whitespace before and after ‘<?php ?>’ in wp-config.php in wp-db.php and post.php but this doesn’t help.
Any Idea?