BuddyPress.org

Headers Already Sent out errors are almost always caused by blank spaces in the PHP file, check that you don’t have ANY extra spaces in the files first ?php by mistakenly pressing enter. You can compare so see if you have by downloading buddypress and looking at the original file

Or, an easier way, is to just copy the original file from buddypress. (manually download it on the site)

Copy the bp-messages.php and ajax.php and paste them into your directories