An error such as the following is produced in the php error log:
(WHMCS\\Exception\\Module\\GeneralError(code: 0): Permission Denied at /home/username/public_html/modules/servers/cpanel/cpanel.php
The disk usage for users are not updating properly in the WHMCS interface.
Permission denied errors such as this typically stem from API access errors.
Check to ensure that the API Token that you are using to authenticate with has the correct permissions and is valid.
Check for cPanel and WHM plugins that could be restricting access.
One example is the Root Restrict plugin. If you have the Root Restrict plugin installed, check to see if your WHMCS server's IP address is whitelisted.