There are 3 possible reasons why you get the notice:
1. Too many slides
You added too many slides, like 40, 50 or above. When this happens, it means that some of the server's variables are set to low values and you will need to increase them. These variables can be found in the php.ini file, in case you have access to it. If not, please contact your hosting provider and ask them to increase the values for you. These are the variables that need to be increased:
- suhosin.request.max_vars and suhosin.post.max_vars, if the server has suhosin installed
2. HTML form element inside the slide
The slider's admin interface itself is a form; having a form inside another form is not allowed and will cause problems. A workaround for this situation is to add the form using a shortcode.
3. XML-DOM library is not installed
This potential problem is more difficult to detect, but when none of the above situations apply, you can consider this problem. Your hosting provider will be able to confirm if the XML-DOM library is installed. This library is used by the plugin to save each slider in an XML format, which allows you to export and import sliders. You can disable this feature, if you want, from within the Slider PRO -> Plugin Options page.