Warning: session_commit() [function.session-commit]: write failed: No space left on device (28) in /home/username/ public_html/CMS/Class/auth.php on line 238

Warning: session_commit() [function.session-commit]: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in /home/username/public_html/CMS/Class/auth.php on line 238

This causes of warning message to be generated for  php unable to write session data as web server has run out of space So that the query commands cannot be executed. Its necessary  to check your occupied disk space using df command with -h.

# df -h

Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5             910G  652G  213G  76% /
tmpfs                 3.9G     0  3.9G   0% /dev/shm
/dev/sda1             194M   77M  107M  42% /boot
/dev/sda2             5.0G  4.9G  0    100% /tmp

Clear your tmp directory and refresh of the browser page

I assume that the web hosting has set space for your account which you have exceeded, clean unwanted files from there or contact to hosting provider to fix this Warning Messages.