Are you using custom authentication? If so, the problem might be related to session timeout or to auto-logout functionality.
See http://www.tersus.com/#Id=1595 for a discussion of session timeouts.
Auto-logout is a feature that terminates a user's session when the user leaves the application. the problem with it is that it also terminates the session if the user opens the same application in several windows and closes one of the windows. Auto-logout can be disabled by putting tersus.autoLogout = false; in web/settings.js
For best results, use the Firefox browser..