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To resolve above error refer following steps.
root@server[~]#cd /var/cPanel/overquota/
root@server [/var/cpanel/overquota]#ll | grep your_cpanel_user_name
Once you found out file for your cPanel user name either remove the file or rename it like cpanel_user_name-back etc.
And now browse your webmail but make sure that you have increased disk space for your domain if its really exceeded.
Thanks 
