PHP Fatal error: Class 'vz' not found

[aksteven]

I have rebooted my server after numerous problems that each require a separate thread. I have just received three system email as follows:

/usr/local/psa/libexec/modules/watchdog/cp/pack-sysstats day
PHP Fatal error: Class 'vz' not found in /usr/local/psa/admin/plib/PleskException.php on line 22

/usr/local/psa/libexec/modules/watchdog/cp/clean-events
Same message

/usr/local/psa/libexec/modules/watchdog/cp/clean-sysstats
Same message

Plesk panel version: 11.0.9 Update #46, last updated at April 13, 2013 04:05 AM
OS: CentOS 5.6 (Final)

Any ideas?