Problem posting

I’m experiencing a problem trying to post a text to the topic session of LessWrong discussion.

When I write a small text (like this one) it works fine.

When I copy part of the text I intend to publish, and try submitting, still fine.

But if I copy most of it, or the entire thing, when I click submit the red sign saying “submitting” appears for a few seconds, then disappears, and nothing happens.

Does anyone know what happened or how to solve it?