That dark screen is an overlay added by your theme. It appears to be triggered by the #hash added to the url after form submission.
Unfortunately, that means the plugin is working as expected and the issue lies with your theme (it possibly another plugin).
If you want additional assistance, premium support is available, but free support does not include troubleshooting theme issues like this.
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This reply was modified 7 years, 3 months ago by
Nick Ciske.
How do I get premium support? Thanks
Premium Support (paid)
Timely support directly from the plugin author!
* Premium support includes help troubleshooting plugin and theme conflicts, customization of the form, extra validation and other issues not generally covered by free support.
* We’ll answer your question and work to solve your issue in the timeframe you choose.
* Depending on the amount of work involved we’ll either fix the issue, or get you an estimated cost and timeframe for a fix.
Get Premium Support
https://luminfire.com/support/premium-plugin-support/