Even though I am supposed to be logged in "forever", I found myself unexpectedly logged out when I first visited the forum during the current session. So I logged in forever again. But, every time I return to the Index page from anywhere else in the forum, I get kicked out. This has just happened at least 10 times in as many minutes.
This sounds as though we may have found a bug in the software. I have done some computer maintenance today, which always logs me out, so I haven't noticed that yet. I have also logged in forever but will not see anything until tomorrow. If it happens to me, I will check at the support forum to see if they know anything about it.
The only other answer is that the cookie placed on your computer so that the forum software can recognise you has corrupted. One way of dealing with this is to delete your cookies and then login to the forum again, selecting "Forever" and see what happens.
I have remained successfully logged in for several days now but I found myself well and truly logged out tonight. But I noticed a difference this time.....
The home page was inviting me to either log in or register so, I clicked on "login". As soon as I arrived on the Login page, it said "Hey, Roger, you have 0 messages". No change there, then - nobody talks to me anyway! But at least I was already logged in. As soon as I returned to the home page, I was logged out again and so on.