nBill is not capable of producing a forbidden error itself - that error comes from the web server software. So there is a conflict between the server configuration and what nBill is trying to do (which is basically just overwrite its own files). That's not to say there is an error in the server configuration, just that there is a conflict between the two (but that being the case, if you want to use that feature of nBill, and your server configuration does not allow it, you would need to change your server configuration - or in this case just use an alternative method of upgrading).
If you send the error report to
support@nbill.co.uk though, I will see if it sheds any further light on it.