error 4401: Unable to populate data, please check if mandatory value is entered correctly.

error 4401: Unable to populate data, please check if mandatory value is entered correctly.

I have only 1 mandatory field, I tried the following xml:

var xml = "<Contacts><row no=\"1\"><FL val=\"Last Name\">last</FL></row></Contacts>";

var xml = "<Contacts><row no=\"1\"><FL val=\"Last_Name\">last</FL></row></Contacts>";

var xml = "<Contacts><row no='1'><FL val='Last Name'>last</FL></row></Contacts>";

var xml = "<Contacts><row no='1'><FL val='Last_Name'>last</FL></row></Contacts>";

URL:

https://crm.zoho.com/crm/private/xml/Contacts/insertRecords?scope=crmapi&authtoken=XXXXXXXXXXX&newFormat=1&xmlData=<Contacts><row no='1'><FL val='שם_משפחה'>last</FL></row></Contacts>

any idea?