Sorry, I was unclear.
I go into a thread in the 4Ed forum to see what someone posted...maybe post something myself. When I try to go back to the forum from which I entered the thread, I get the error message.
IOW, I'm not jumping to another forum, I'm basically backtracking.
Further refinement: it ONLY seems to happen when I try to get to the 4Ed forum via the forum jump feature. If I just use the "back arrow" in my navigator, it seems to be just fine.
But if I use the jump feature, it won't let me back into the 4Ed forum unless I go to General RPG Forums page first, and enter the 4Ed forum from that screen.
And I'm not confused by the reshuffled forums- only one single 4Ed option is being shown on the forum jump menu- there's no way to choose am incorrect 4Ed forum.
Refinement #2: it seems I can't use the jump menu to get to the 4Ed forum from anywhere. So, for instance, if I try to jump from here to there, I get the error message.