Problem inserting invoice

Problem inserting invoice

Trying to insert an invoice into Zoho CRM but always getting a response of:

<response uri="/crm/private/xml/Invoices/insertRecords"><error><code>4500</code><message>Problem occured while processing the request</message></error></response>

which is not that helpful. I would gratly appreciate guidance as to where this message format is wrong.
Also, finding it confusing about when field names have spaces in them or not eg. "Product Details"

url: 
https://crm.zoho.com/crm/private/xml/Invoices/insertRecords

parameters:
authtoken=mytoken&scope=crmapi&newFormat=1&xmlData=
<Invoices><row no="1">
<FL val="Subject">99999</FL>
<FL val="ACCOUNTID">342698000000262005</FL>
<FL val="Account Name">Acuere</FL>
<FL val="Due Date">11/21/14</FL>
<FL val="Invoice Date">11/07/14</FL>
<ProductDetails>
<product no="1">
<FL val="Product Id">342698000000113001</FL>
<FL val="Product Name">BizLinker - AO to MYOB</FL>
<FL val="Quantity">1</FL>
<FL val="List Price">0</FL>
<FL val="Tax">10</FL>
<FL val="Total">100</FL
><FL val="Discount">0</FL>
<FL val="Total After Discount">100</FL>
</product>
</ProductDetails>
</row>
</Invoices>