Have you tried putting any other widgets in there, like a Text Widget, just to test it?
What plugin provides the subscriber widget? I looked at the source code to see if there is anything in the sidebar above the Amazon ad and I don't see anything there. That tells me that WordPress isn't trying to put out the code and failing.
Make sure you're adding the widget into the proper sidebar. If you see the widget for the Amazon ad then you're probably in the right place, but there could be more than 1 sidebar that is being shown based on the page/post style. Check all of the widget areas to see if there is another one that is being used. I doubt this would be a problem but it doesn't hurt to check.
It's entirely possible that this may be caused by something completely different though. Maybe there are plugin conflicts or a problem with the database.