What are some possible causes of error message "javax.xml.soap.SOAPException: Bad response: 401 Unauthorized" encountered intermittently

I have a web service that when I invoke, I encounter a "javax.xml.soap.SOAPException: Bad response: 401 Unauthorized." error. This happens intermittently. When I restart the server, the web service is invoked successfully, but after a few minutes, the error is encountered again when the same web service (or a different one) is invoked. I know that error 401 is encountered whenever the client request did not pass the authentication details/passed incorrect authentication details. I just don't understand why this happens intermittently and why it works whenever I restart the server (which I am not always supposed to do). Any idea why?