It seems that Ninjaforms in ver. 3 does not work with your theme. Thats what the Ninja-Support Team wrote me:
"It appears that something in your theme is breaking the form submission. When the theme is switched to Twenty Sixteen, it submits normally. It looks like your theme’s lighbox gallery and/or some Google Analytics functionality are broken, causing the JavaScript error that is preventing the form submission in Safari. I do not know why it doesn’t impact the other browsers, but your next step should be contacting your theme developer with the above information and they should be able to assist you further."
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Now beside comment from other support team, please write what is the case with link to page. Are you new to our support forum?;-)
With kind regards.