FWIW, if you are posting a reply and you see Service Unavailable instantly, it didn’t go through, and you can try again.
If it delays a while and then says Service Unavailable, it means that your post went through but you didn’t get the next step of accessing the thread.
So, go back to Post, save your message in case it isn’t there for some reason, get back in the thread again, and you should see your message there, if there was a delay.
That way, you don’t add to all this multiple posting.
Well, that method appears to pare it down to two.