Ninja Forms Enquiry Button

We’re moving to a new theme (Extra) and are running through tests in a staging environment. Everything appears to be working ok with GeoDirectory, but we have hit a snag with the Ninja Form Contact Form. It was working ok in the old theme (using Sidebar widget GD > Ninja Forms), but is no longer working properly. Now when you click on the button in the sidebar, it starts loading the lightbox popup – you see white spinning arrows with a black screen, but nothing further happens and the form does not display.

I’ve tried setting up the contact form in a few different methods, as described in your documentation
https://wpgeodirectory.com/docs-v2/integrations/ninja-forms/
– GD > Ninja Forms Widget
– Shortcode: Lightbox Button on Sidebar
– Shortcode: Lightbox Button on Detail page

but get the same problem with all of these methods.

The following console error displays:
admin-ajax.php:1 Failed to load resource: the server responded with a status of 500 ()

We would really appreciate any assistance you can offer. Thanks.