'/var/lib/php/session' are being set to 0755 after each microupdate installation.
Several Plesk 11.0.9 micro updates bring up newer PHP version, that reset permissions on "/var/lib/php/session" and sometimes micro-update script doesn't set it back. This issues were reported to Plesk Service team as bug TP#131695, and was fixed by update script in MicroUpdate #42 .
Install the laest Plesk microupdate:
/usr/local/psa/admin/bin/autoinstaller --select-product-id plesk --select-release-current --reinstall-patch --install-component base