I kept receiving intermissive PHP warnings saying some thing like E_WARNING: session_start(): open(/var/lib/php/session/sess_ji9k4chqke3pde98a5n5m1vca5, O_RDWR) failed: No space left on device (28). Usually this would indicate that the disk where the sessions were being stored is full. The best place to start is to check if the disk where those session files are being stored is in fact NOT full.
// to check disk space usage df -h // should display something like this Filesystem Size Used Avail Use% Mounted on /dev/md1 4.0G 883M 3.2G 22% / /dev/mapper/vg00-usr 4.0G 1.8G 2.0G 49% /usr /dev/mapper/vg00-var 890G 231G 614G 28% /var none 5.9G 116K 5.9G 1% /tmp
In my case the disk where the sessions were being stored had plenty of space - 600GB free!