The problem happens when the disk quota file gets corrupted on the server. You can run the following command while logged into the server as root.
/scripts/fixquotas –force
It may give errors like the following if the problem is not solved completely.
Resetting quota for abcde to 500 M
edquota: Quota write failed (id 32230): Invalid argument
Resetting quota for xyz to 500 M
edquota: Quota write failed (id 32010): Invalid argument
In that case, a quick reboot of the server will help to solve the problem. You may restart the server via SSH using the command.shutdown -r now