Below are the possible reasons to get this error message in WebAgent.log file on Web agent side.
1. If the Agent receives the request with an expired session cookie. Check if the browser is presenting expired session cookies.
2. If the user refreshed the browser after the cookie has expired.
3. If the machines in the environments are out of time sync ( Both date & time). Also check the timezone of the machines.
4. If we are using any health check tools for checking the health of web servers, by accessing a particular heart beat url.