The problem is that an active session loses an attribute without explicitly removing it by setting it to null. Here's how and when it happens: An apache web server runs an index html file. This file has a link to a servlet: upon clicking, the call is redirected to a second, Tomcat server that hangs behind the apache server and cannot be called directly itself.
The servlet creates a session and sets an attribute. When you return to the index file and then visit the servlet again, the session is preserved, but the attribute gets lost. Has anyone got a clue? Thanks, Marc