/var/lib/php/session folder containing PHP sessions has a large number of files.
/etc/cron.hourly/plesk-php-cleanuper, CPU consumption increases to 100%.
When attempting to find all working PHP session processes, this fails with error
Too many open files.
Internal software issue with ID #PPPM-2316. It was fixed in Plesk 12 microupdate #44.
For Plesk 12 apply the latest Plesk microupdates.