I've had the same issue - the problem occurred because an empty line / whitespace was output in one of the template files BEFORE the headers get sent.

Search for empty lines which arent enclosed by <?php ... ?>. My file was corrupt because it started with an empty line, then came the <?php ... but it can also be in the middle of a file, or at the end.

EDIT: Oh wait, I just saw that you're talking about IPB, and I've had this issue with WP, but the cause of "headers already sent" message is probably the same.