wp-admin ajax failing because of newline in beginning of xml response

When I’m using the dashboard, for example, adding a category. The request returns valid XML as expected, but it is prepended with a newline character, causing the DOMParser() to fail.

I’m not sure where to proceed for debugging, it happens on one site and not another (different server, different theme). I’ve checked, none of the theme’s php files have any characters proceeding <?php and none end with ?>

Any suggestions?