This error is returned if the Customer Record in Xero has a non-valid Website.
New validation has been added to the Customer Contact which could allow a invalid URL (previously saved) to stop a record from being created with this record.
To correct this error, find the customer contact in Xero for the invoice which is erroring. You will note that the website is not valid.
Correct the website by either removing or updating the record, Press Save.
The record will now sync without further error.