Error message: [Order number] Invoice not of valid status for modification

An error message like the below:

	100001012: Export did not succeed
	Errors received from Xero: A validation exception occurred
	Invoice not of valid status for modification

usually indicates that:

1. Xero already has this invoice number (from Magento or other source) and
2. and a payment has been applied against it.

Any further changes to that invoice are then prevented by Xero with the above error message. We suggest taking a look at the invoice history in Xero (see below) to establish if this invoice had already been exported by our integration, was possibly exported wrongly from a test site (we suggest connecting any test/staging/dev sites to a Xero demo organisation to prevent this from happening) or is genuinely a double up of order numbers.