Lead Converstion Problem

Lead Converstion Problem

 Hai All,

I am trying to convert a lead  to potential with the URL:

http://crm.zoho.com/crm/private/xml/Leads/convertLead?apikey=+APIKEY+&ticket=TICKET&leadId=entityId&xmlData=POTENTIALXMLDATA

when i run this application it giving response as:

<?xml version="1.0" encoding="UTF-8" ?>

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



My account is free . Please anyone advise me how to convert lead to potential.

I need information what is mean by qualified lead and how to change the privileges.

Thank you