I've finally found out what is wrong . There was a missing tag in the XML.
sb.AppendLine("<s:Envelope xmlns:s=\"http://schemas.xmlsoap.org/soap/envelope/\">");
sb.AppendLine("<s:Body s:encodingStyle=\"http://schemas.xmlsoap.org/soap/encoding/\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">");
sb.AppendLine("<q1:runReport xmlns:q1=\"http://axis2.ws.jasperserver.jaspersoft.com\">");
sb.AppendLine("<requestXmlString xsi:type=\"xsd:string\">");
sb.AppendLine("<![CDATA[<request operationName=\"runReport\" locale=\"br\">");
sb.AppendLine("<argument name=\"RUN_OUTPUT_FORMAT\">HTML</argument>");
sb.AppendFormat("<resourceDescriptor name=\"Accounts Report\" wsType=\"reportUnit\" uriString=\"/reports/samples/AllAccounts\" isNew=\"false\">");
sb.AppendLine("</resourceDescriptor>");
sb.AppendLine("</request>]]>");
sb.AppendLine("</requestXmlString>");
sb.AppendLine("</q1:runReport>");
sb.AppendLine("</s:Body></s:Envelope>");