Currently your messages log is filling up with errors and the system is unstable. Check the message log:
$ server# tail -f /var/log/messages Jul 28 08:57:30 mail kernel: EXT4-fs warning (device sda3): ext4_dx_add_entry: Directory index full!
Check the sessions directory
$ server# php -i | grep session.save_path session.save_path => /var/lib/php/session => /var/lib/php/session
$ server# du -shcx /var/lib/php/session 1000.0M /var/lib/php/session 1000.0M total
A cron job similar to the following ran every day, or periodically, should prevent those files from accumulating.
# find /var/lib/php/session -type d -mtime -15 -delete
Manually
# find /var/lib/php/session -depth -mindepth 1 -maxdepth 1 -type f -cmin +120 -delete;