error: cannot modify header information!?

Hello there!

I have a small problem: everytime I try to edit a post and re-publish it, I get the same error message:

“Warning: Cannot modify header information – headers already sent in C:\apachefriends\xampp\htdocs\wordpress\wp-admin\post.php on line 122”

Can someone tell me what this means?
Maybe you need to know that I am testing wordpress with the “kiwi” theme.

Thanks a lot
Yours!