I had the same problem and it was a post that I had cut and paste a couple of links into.
The links were not attached using the "insert link" icon at the top of the post editor. I chalked it up to that, although it may have been the link location too, because it was to some other posts in mc forums. I re-posted without the links and it posted immediately, so I think that's the issue. I'm sure it's automated so I'm not sure if the cause is a url blacklist, or url links not wrapped in html or what.