This error message means that the XML is incompatible or does not follow the invoice format rules.
A validation error can occur e.g. because a text in an element is too long or exceeds the character limit.
You can test your XML-file at your Maventa account under Invoices -> Test XML invoice material. The validator tells you which row in the XML has an error, as well as a description of the error. The XML should be corrected in your ERP and resent.