[support] how to reset the visit counter? always 125 visitors online!

Hi, I have a serious problem with my d6.15, because yesterday our service provider made some strange things with date, they installed a plesk update that caused problems to many users. In particular, my problem is that the drupal counter is always showing me that there are 125/140 anonymous visitors, that is simply impossible! and, cron doesn't start any more, I have poorman's cron module, and I must run cron.php in order to run it properly. Any hint on how reset the visit counter? I tried to clean the cache, to put website in offline mode and then online, run update.php, nothing1 always 130 visitors!