"Warning: cannot modify headers" error


I get:
Warning: Cannot modify header information - headers already sent by (output started at xxx/web/wp/wp-includes/functions.php:3995) in /xxx/web/wp/wp-includes/pluggable.php on line 1174

I cant make any sense of this error, since I havent touched the files mentioned in the error, for me to try to fix a possible error in i.e functions.php could possibly make stuff worse I guess.

Any suggestions?