Hello and welcome to the Community, pete243
Thanks for letting us know that you're experiencing this issue. Rest assured; this has been reported to our higher support team who're working to uncover the cause.
At the moment, the error appears to be intermittent and may self-resolve by creating a new invoice.
If you would like to be added to our internal investigation for all updates, please contact our support team directly quoting the reference INV-103515. Thank you.