Header already sent

I just started working with WP. I had an old site at http://sweetaudio.nl, I made a new site at http://sweetaudio.nl/wp. I had it al working, and then I wanted to replace the old with the new. I moved all folder in the FTP to new map called ‘oldsite’ (except the wp map and .htaccess) and copied the index.php from wp to the root. And changed in the index.php in the root the header location.

I can still login, I can see the site. But I can’t change anything… I get these errors:

Warning: Cannot modify header information – headers already sent by (output started at /www/htdocs/sweetaud/wp/wp-includes/functions.php:1) in /www/htdocs/sweetaud/wp/wp-admin/post.php on line 229

Warning: Cannot modify header information – headers already sent by (output started at /www/htdocs/sweetaud/wp/wp-includes/functions.php:1) in /www/htdocs/sweetaud/wp/wp-includes/pluggable.php on line 896

I read already that it probably has something to do with spaces before or after <?php and ?>, but I triple-checked and triple-checked that again.

I’ve no idea what to do next..

Can someone give me some advise?