I'd log the ticket with Appnitro, they will probably ask you to provide your hosting account detail etc so they can they can check it.
You say "at submission of the form" this error occurs for one form only?
The process of elimination:
a) does the form use the email notification (is the problem with the email address that is being sent the email either the admin or the user notification)
b) conditional logic (are all the rules correct)
c) Captcha
d) file upload (file sizes within the server limits etc)
A server "500" error is generic and not a specific problem, you might also have to dig into the server logs to find a report of the problem. But that's something Appnitro can help with from their side .
You could also turn on PHP error reporting through your hosting account and see if you get any more details of the problem.
