Question

My current properties:
Session.Timeout = 500; – ignored;
On the server properties – Session State Time-out = 500 – didn’t help.

Answer 1

Have you set the session timeout in the web.config file?

<configuration>
  <system.web>
     <sessionState timeout="500"></sessionState>
  </system.web>
</configuration>

You might also want to have a look this site for other reasons: http://patelshailesh.com/index.php/session-expires-before-timeout-value-specified-in-web-config

As @Hans has suggested, you can also use a “keep alive” mechanism instead of the sledgehammer method of increasing the session.
Here’s one: “session defibrillator”