I tried it out just now on my computer (Delicious Post 1.5, Firefox 19 beta, Windows XP), and I'm not getting such an error when posting a URL to my personal Delicious account. I navigated to my Delicious page and can confirm that the URL I had posted appears in my list of links. I'm also unable to reproduce the problem on my Mac: Delicious Post 1.5, Firefox 18, Mac OS X 10.7.5.
Perhaps this was a temporary problem on the Delicious site; please give it another try.
Alex