Unknown exception, internal system processing error

 we receive the following error when we try to call  WebService without security header after configuring the WS security policy:
<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
   <env:Header/>
   <env:Body>
      <env:Fault>
         <faultcode>env:Server</faultcode>
         <faultstring>Unknown exception, internal system processing error.</faultstring>
      </env:Fault>
   </env:Body>
</env:Envelope>

There are no errors in application logs.When we do include the security header, then the webservice works as expected.

Please advise if there is a setting that we may have missed.