I've recently chosen XOOPS over postnuke and phpNuke for my guild site; a big step up from HTML. All was well until members wanted to create accounts and login. For me, when I log in, all is fine, but, when anyone else logs in, they are logged in until they change the page, then they are instantly logged out. On the "Whos Online" module it displays them as logged in and also as a guest (identical IP) and the guest always stays in the frontpage or news. It also shows me as a guest (identical IP) as well as logged in, except I STAY logged into my account.

Has anyone had this error and/or know how to fix it? I would really like to get it fixed ASAP. Also, in Admin CP it tells me my mainfile.php needs to get its access permissions fixed, in a red box with WARNING. How do I fix this and could this cause the login errors?

Things I've tried:
Custom session: Timeout 1hr
SSL Login
SSL+Custom Session
Neither of the two