It's probably caches or cookies, have you tried it again? it should be not a big deal because sometime it also happened to me, I'm just refresh the page and click it again, there's no more such errors.
Looking at your rank and your activity, it seems you're clicking enter too fast when you want to make a post because newbie rank need to wait for few minutes between each post.
i tried doing it twice and it still showed the same message. i don't think its because of that because it didn't even let me go to the page where i can type what i want to post. it just goes on another page with that message on it.