The issue has been fixed since Plesk 9.5.3.
Parallels Plesk Panel does not update traffic statistics for clients. When you click any client on Plesk server the clients traffic bar on the main page shows 0 traffic, though the individual domains traffic are calculated correctly.
The problem is that none of the client traffic bar is showing any traffic.
The problem in that DailyMaintainance script loses MySQL database connection during long time execution.
Increase value of wait_timeout in /etc/my.cnf cofiguration file to 28800 and run DailyMaintainance script:
# /usr/local/psa/bin/sw-engine-pleskrun /usr/local/psa/admin/plib/DailyMaintainance/script.php