[eluser]WanWizard[/eluser]
I thought so.
By default, the session class rotates the session_id every 300 seconds (is also a setting in the config), if this happens on an ajax call, the session_id is updated but no updated cookie is send to the client. So at the next page request the session_id no longer matches, and you are logged out.
The solution to this has been discussed before on this forum. Search for sess_update and IS_AJAX.