I am using SOAPUI to post an XML. I have the below XML code: 

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:enterprise.soap.sforce.com" xmlns:urn1="urn:sobject.enterprise.soap.sforce.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> soapenv:Header urn:OwnerChangeOptions urn:options urn:typeKeepAccountTeam</urn:type> urn:execute1</urn:execute> </urn:options> </urn:OwnerChangeOptions> urn:SessionHeader urn:sessionId</urn:sessionId> </urn:SessionHeader> </soapenv:Header> soapenv:Body urn:upsert urn:externalIDFieldNameSAP_No__c</urn:externalIDFieldName> <urn:sObjects xsi:type="urn1:Account"> urn1:SAP_No__c006851xxx</urn1:SAP_No__c> urn1:Customer_Service_Z6__r urn1:SAP_User_Id__cDExx</urn1:SAP_User_Id__c> </urn1:Customer_Service_Z6__r> urn1:Owner urn1:SAP_User_Id__c00000xxxx</urn1:SAP_User_Id__c> </urn1:Owner> urn1:Sales_Rep_SAP_Z1__r urn1:SAP_User_Id__c00000xxxx</urn1:SAP_User_Id__c> </urn1:Sales_Rep_SAP_Z1__r> </urn:sObjects> <urn:sObjects xsi:type="urn1:Account"> urn1:SAP_No__c00532xxxx</urn1:SAP_No__c> urn1:Customer_Service_Z6__r urn1:SAP_User_Id__cSKxx</urn1:SAP_User_Id__c> </urn1:Customer_Service_Z6__r> urn1:Owner urn1:SAP_User_Id__cV2xxxxx</urn1:SAP_User_Id__c> </urn1:Owner> urn1:Sales_Rep_SAP_Z1__r urn1:SAP_User_Id__cV253xxx</urn1:SAP_User_Id__c> </urn1:Sales_Rep_SAP_Z1__r> </urn:sObjects> </urn:upsert> </soapenv:Body> </soapenv:Envelope>

I am getting the following response:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns="urn:enterprise.soap.sforce.com"> soapenv:Header 20018 5000000 API REQUESTS </soapenv:Header> soapenv:Body false OwnerId All accounts must have the same current owner and new owner. INVALID_FIELD 0013N0xxx0pnZxxxAU false false OwnerId All accounts must have the same current owner and new owner. INVALID_FIELD 0013N00xxxxoQAI false </soapenv:Body> </soapenv:Envelope>

Please let me know for any solution to resolve the issue