Whitespace before or after the PHP tags.
WHAT IT MEANS
Output was sent before WordPress could set headers. Almost always a stray space or blank line.
WHERE TO LOOK
The error names the file that produced output. That file is the problem, not the one attempting the header.
The usual culprits are wp-config.php and a theme's functions.php.
THE FIX
Open the named file.
Check the very beginning: there must be nothing before the opening tag. Not a space, not a blank line.
Check the very end: remove the closing tag entirely. It is not required in a pure PHP file, and removing it prevents trailing whitespace causing this.
WHY IT HAPPENS
An editor adding a newline on save Text pasted with a leading space A byte order mark added invisibly by some editors
IF YOU JUST EDITED THE FILE
Restore your backup copy and edit again more carefully.
CHECKING FOR INVISIBLE CHARACTERS
Some editors show them. Alternatively, retype the first and last lines rather than trying to spot the problem.
