You’re absolutely right. I disabled AdBlock (2.41.2) and it worked!
I’m not 100% sure how AdBlock works, although given that the topic of this post is about Advertising that would make sense.
Thanks!
EDIT:
I replicated this in try.discourse by creating a post with the word “advertising” in the subject. Initial post creation is fine but future saves initiate a post back to the friendly URL Discourse creates (http://try.discourse.org/t/advertising-test/329). I think AdBlock simply works on the basis of inspecting and blocking HTTP calls, so there’s a rule there that will block XHR calls if the word “advertising” is found in the URL.
Not really a Discourse issue but it may be useful for anyone else encountering the same behaviour.