When you login to webmail, it might show you an error like above. To correct the issue it is necessary to increase the defined disk quota of the user account. Make sure the email account and the cPanel account as a whole have sufficient space. However, there are times you might see an error even if the accounts are under the limits. In that case, try the below solution:
Make sure you replace the username with the actual cPanel username.
The error should disappear when you login to Webmail the next time.