Do you have a test account for use? I can see your main page and the Currently Active Users list looks fine, but I can't look at the Who's Online list. (Dirty_Sanchez is a user on your site? Hmmmm

)
Leave the session timeout as 900 and don't touch your cookie domain or path as that is all working fine on your site.
The IP thing is something I think you need to ask your host about.