WPMU 2.6.2 with Atahualpa 2.21

Check the How do I solve the Headers already sent warning problem? question in Codex.

Other resource:
https://mu.www.ads-software.com/forums/

Check both /wp-content/themes/atahualpa2/functions.php and /wp-login.php for blank spaces or blank lines after the last closing ?> tag, or blank lines anywhere outside of <?php ... ?> tags.

Thank you both. It appears that one of my plugins altered /wp-content/themes/atahualpa2/functions.php adding a blank line after the final ?> tag – resolved.

Here is an update. I did find several blank spaces on line#571 on the functions.php in the original download of the atahualpa2 theme. I don’t recall where I downloaded the theme (WP.com or the theme site).

If the blank lines are inside of opening <?php and closing ?> php tags then it’s fine.