"Preview before submit" does not work ans just says "Please wait.."

I am also having the same issue across multiple forms.  Since we use dozens of forms your suggested fix is not an ideal choice.  It appears there is a bug in your system.

Please try updating the widget on the forms and it should be working fine.

I have updated the widget on the form http://www.jotform.us/form/52316953446156 in your account and see that the preview is working fine now. 

Please give it a try and let us know if you need any further assistance. We will be happy to assist.