If you have upgraded you have not uploaded all the proper new files. Make sure to reupload the files except the config files from the latest downloaded code.
set_user_perms is indeed in pp-inc.php and tells me you have uploaded your new forum integration file but not the new pp-inc.php file. That tells me you have some new some old files on your upgraded site. Make sure as well you run and required upgrade you need to if you have not done this also.