Error message :

"Invalid Maildirsize File"

when attempting to try and change the email account quota in your cpanel.

Here is a solution that will fix it.

1. Login to your cpanel

2. File manger :: Browser over to the "mail" folder which you will see The full path would be:
/mail/mydomain.com/.me@mydomain.com (mydomain.com being your actual domain name)

3. Now you can delete or rename the following file : "maildirsize"

4. Once it's deleted, login to your cpanel account and try to change the quotas. It will then reform itself automatically and re-create the maildirsize file correctly.

Now you will be able to change quotas for your email account(s).