Can you try another browser on the same computer (just want to confirm that it's all browsers)
Also instead of using quick reply, try clicking on Go Advanced and posting via that method. See if it happens when you do that.
It's happened once before here and I reported along with many others to vBulletin. Unfortunately they believe there is nothing wrong. It's really hard to pin it down and identify the issue when it is only happening to some users some of the time. It isn't happening to me but I have no doubts that some of you are having the issue.
Just to be sure, you definately aren't pressing the Post Quick Reply button twice (ie you click once and it starts moving but goes slow, then you click the button again and it comes up with the error message you are getting) ??