When you are trying to save on your slider (or slide) and you cannot, but just an Unexpected response error message will appear in a lightboxed window, that means there is an issue with the server. They have some limitation or misconfiguration, which doesn't allow your settings to be saved. You should get in touch with your server host and ask them to help you resolve this problem. Most commonly a ModSecurity setting causes the problem, but they should have a log about what is happening for what reason on your website, which could point out what limitation causes the issue. This error can only be fixed by your server host.