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.
deliteblogger Reviewed by deliteblogger on . [IPB 3] PHP Errors and Sing-in Problems Hey, I have a problem again, actually it occurred afterwards the update from 3.1.4 to 3.2.3 I`m using the raw IPB, without any file modification. The problem I have is I`m getting auto-logged out, and auto-login too. Once after logging in, and trying to browse any topic, I`m auto logged out. And even after closing the browser and coming back after sometime, I`m still logged and chain continues. My cPanel error log is empty. Rating: 5