Your inodes are full, so there's files somewhere in your server occupying them.
To find what's consuming inode space, run this:
for i in /*; do echo $i; find $i |wc -l; done
After it returns the sizes of each item, look through them and see which directory is the biggest. Keep drilling down directories a few times until you found the culprit folder. It's probably log files (error logs/admin logs), PHP session files, or in my case, mod cache disk. I had to keep going into the /var folder, show below.
for i in /var/*; do echo $i; find $i |wc -l; done
Once my search showed that /var/cache/apache2/modcachedisk was using almost 100% of my inodes, i deleted the folder and restarted apache and the server, making my inode usage only 4%, down from 100%.
service apache2 restart