Solution 1: Please log in to your Pinterest account. Confirm possible confirmation queries and change your password if it is requested by Pinterest.
Solution 2: Pinterest has blocked all links made with a url shortner. For details, please see the official statement of Pinterest "Blocked links and websites". We have already adapted those changes at Blog2Social. By now, Blog2Social uses the original link to your post. This way, the plugin works according to the guidelines of Pinterest.