Through experimentation and investigation, I have determined what the issue is. The issue with StumbleUpon sharing lies with URL shorteners. If you enable the built-in wp.me short links or set up bit.ly short links, you’ll get the Add Page Error when you try to share to StumbleUpon. If you disable short links, then the StumbleUpon button/badge will share pages just fine. Unfortunately, this means you cannot use short links for any social sharing. I don’t know if there is a way to allow users to choose which social media sharing options they want short links used with.
