I *think* this has to do with your items specifics. We can pinpoint things better if you file a support case and tell us the title of the auction that won't load.
However, off the top of your head to your remember if you chose item specifics for one of the categories you are listing the item into? There seems to be a way to pick item specifics, but not finish choosing all of the item attributes, and that breaks the ad page.
If that is the issue you are having we are working on a fix, but it won't be live until tomorrow.
If you don't need to edit the listing before posting it, just select "schedule" from the saved listings page. That should circumvent the problem until we get a fix in there.