I seem to have the same issue. Sometimes I enter and am fine and other times when I try to post, I have to log in. Seems to me to be inconsistent but maybe there is a method to the madness.
I just figured out if I got $100 every time I got this message after having to re-log in...
"The forum you selected does not exist."
...I could buy Strat-O-Matic and fix the forum so it doesn't randomly log you out or give you this bogus message.
Then again, if they got $1000 every time I said hell with it and didn't make a post because I had gotten logged out, they could buy their company back!