As I said on the other thread: "Can't Post... Do This.., it may well be an automatic protection built into the system to prevent the forums from overloading and possibly crashing, and to prevent overall site slow-down.
That's just a guess on my part, but it makes sense. It could be a matter of waiting 20 - 30 minutes, perhaps even less in some instances, then trying again.
Hopefully a dev or mod will come along to clarify this for us.