The filesystem mounted at / on this server is running out of disk space

From CPanelDirect

cPanel has built in protection against / getting too full. The actual cpanel control panel functions will stop to prevent problems from space filling up. WHM will continue to work. The general error will look as follows:

Sorry for the inconvenience!
The filesystem mounted at / on this server is running out of disk space. cPanel 
operations have been temporarily suspended to prevent something bad from happening. 
Please ask your system admin to remove any files not in use on that partition.

In many cases you should consider adding another harddrive, or if you have second harddrive start moving accounts between them. However there are times when there is data you do not need or log files that can be removed.

First check /var/log as that tends to fill up quickly. Using the du command you can get the space usage, for instance you can run

du -hcs * | grep G

This command will show any files using more than 1 GB in size.

This will work on servers with GNU coreutils installed. On FreeBSD these are not installed by default but can be installed from ports. If you have a lot of files * will eventually not work from too many files. In that case you can do

du -a | sort -n

The file with the most use will be shown last.

In /var/log, it is safe to removing anything ending in .1-4. To quickly do this run

rm -f *.[1-4]

The exim_mainlog tends to fill up quickly as well. If you remove that file exim needs to be restarted with /scripts/restartsrv_exim

The dcpumon folder can fill up quickly too. That is the WHM cpu/memory usage logs so you may not need it either.

Sometimes your provider may leave installd in /root which is the original cpanel installer. It can be safely removed.

In /home there may be un-needed cpmove- files (which are full cpanel backups). Also /home/cptmp and /home/cprestore may have files you do not need.

If none of the above are enough check /usr/local/apache/domlogs and /usr/local/apache/logs for apache log files that may be big. Note: you can set in WHM->tweak settings to remove logs after each stats run.

One final file that may get large in /var/lib/mysql/Your.Server.Hostname.err

If you remove that you need to restart mysql with /scripts/restartsrv_mysql