It sounds like jQuery is being called twice, once by each plugin, resulting in a conflict where one takes precedence over the other. From what I remember, NivoSlider commonly conflicts with several other plugins that use jQuery because it calls its own jQuery, resulting in one plugin just "breaking". Here is NivoSlider's jQuery location on your server:
/wp-content/themes/twenty-plus-pro/inc/js/jquery.nivo.slider.pack.js?ver=3.8
The only solutions I've seen for this type of error either involve use of the WordPress function wp_deregister_script (which I've never used so I don't want to advise you incorrectly on it), but the far more common "solution" seems to be disabling NivoSlider and remaking the slider using a different plugin.
I'm sorry I can't be of more help, but i hope this points you in the right direction.