We tried applying the suggested fix, but without success. In our Photopost version, the update_whoonline function is called from the pp-inc.php file, but is not defined there. It appears to be using the function in the included file, forums\photopost.php. We replaced the original version of update_whoonline in the forums\photopost.php file with the replacement version, but it still displays only the one user.
We're on PhotoPost 5.21, PHP Version 5.1.6. And this worked fine until we moved our whole site to a new dedicated server.