Error 400 Bad Request ( while completing a task)

Hi Experts, I faced a problem with below Error while completing a task, I am not able to get what exactly happened? -<SOAP-ENV:Envelope> -<SOAP-ENV:Body> -<SOAP-ENV:Fault> <faultcode>SOAP-ENV:Server</faultcode> <faultstring>The remote server returned an error: (400) Bad Request.</faultstring> -<detail> <af:legacy_detail>The remote server returned an error: (400) Bad Request.</af:legacy_detail> <af:exception message =" The remote server returned an error: (400) Bad Request. " type ="System.Net.WebException " /> </detail> </SOAP-ENV:Fault> </SOAP-ENV:Body> </SOAP-ENV:Envelope> Help? Regards, Maddy.