Finally found it. Will need to put out a 7.5.4.2 update since this will be an issue for everyone that's running 7.5.4.x.

Line 579 of libs/ubbthreads.inc.php has this:

PHP Code

if ($name == "online_now" && !$online_run) {
  include("{$config['FULL_PATH']}/cache_builders/$name.php");
}

That needs to be changed to this:

PHP Code

if ($name == "online_now" && $online_run) continue;
include("{$config['FULL_PATH']}/cache_builders/$name.php");