Ok, I think I may have found the problem.

I'm testing a fix now.

@session_start() was failing. Pain in the butt to track this down, I had to sprinkle trace commands to find it.

See PHP Bug http://bugs.php.net/bug.php?id=25876

My hosting company is running 4.3.11
I'm running ubbthreads 7.2.2

I'm trying one of the work arounds suggested in the bug

Fix is adding one line of code below to ...\lib\ubbthreads.inc.php
.
.
.
// ----------------------
// Start the session here
if (!defined('ALTER') && !defined('IMPORT')) {
session_save_path($config['SESSION_PATH']);
@ini_set("session.save_handler","files"); // Added per PHP Bug work around http://bugs.php.net/bug.php?id=25876
$check = @session_start();
header("Cache-control: private");
}