The odd thing is I have about seven sub forums. Three of them get this error and the rest do not. I've tried resetting the cache and resubmitting the url and it makes no difference.
The ones with the error seem to validate ok. And they are there when you go there manually. I'm really stumped and concerned (as one of the forums with the error has some of my highest traffic and is most active). I've also disabled the ping, as per a different thread reporting the same problem.