Header is already sent – Error

I’ve done a search here to find the answer on this, but nothing I’ve tried have made any difference.
I’m not new to WP, but this have never happened before.

In the Admin area when I try to unapprove a comment I will get the following error message:

Warning: Cannot modify header information – headers already sent by (output started at /var/www/artfinity/artfinity.co.uk/loeck/wp-admin/admin-header.php:8) in /var/www/artfinity/artfinity.co.uk/loeck/wp-admin/post.php on line 585

I know you guys have seen it before and I know about the whitespace. There is no such thing in admin-header.php and I’ve double checked several times already.
It worked only a couple of weeks ago without any problem, and I have no clue what to do with this.

I can do anything but unapprove a comment.
Please….help…

C.