LTM looses connection with IIS

In a setup where the IIS connction timeout is set to 120 secs (LTM uses 300secs timeout on the oneconnect) the LTM looses connection with the IIS.

That is: In no traffic periods, IIS closes down the connection after 120 secs, but LTM does not discover it, uses it, issues retries and concludes with no data received from IIS. Result: The user experiences a non responding website (page is not loaded). The data is though available when pressing reload in the browser (creating a new connection). When I set the connection timeout in IIS to be > 300 secs, there seems to be no problem. The LTM now closes the connection at 300 secs, not the IIS.

Any idea why this happens ?