Ohhh, I think I see the issue!

Can you tell me the character-count of your post? <http://www.charactercountonline.com/>

I see a "cookie too large" error in the logs. Basically, if there's an error when creating a post (like if it's too long or short), I store the post in a browser cookie and redirect back to the form so you can fix your post and resubmit.

But I now see that max cookie size is 4096 byte (4096 characters). A post can have up to 65536 characters right now. My first suspicion is that you're trying to create a post longer than 65536 characters which is triggering a "post too long" error, but when it saves your post in the cookie, the server is rejecting it because it's too large.