Salesforce "REQUEST_LIMIT_EXCEEDED: ConcurrentRequests (Concurrent API Requests) Limit exceeded."

request_limit_exceeded.xml

<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:sf="urn:fault.partner.soap.sforce.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <soapenv:Body>
        <soapenv:Fault>
            <faultcode>sf:REQUEST_LIMIT_EXCEEDED</faultcode>
            <faultstring>REQUEST_LIMIT_EXCEEDED: ConcurrentRequests (Concurrent API Requests) Limit exceeded.</faultstring>
            <detail>
                <sf:UnexpectedErrorFault xsi:type="sf:UnexpectedErrorFault">
                    <sf:exceptionCode>REQUEST_LIMIT_EXCEEDED</sf:exceptionCode>
                    <sf:exceptionMessage>ConcurrentRequests (Concurrent API Requests) Limit exceeded.</sf:exceptionMessage>
                </sf:UnexpectedErrorFault>
            </detail>
        </soapenv:Fault>
    </soapenv:Body>
</soapenv:Envelope>