404 error

Hi.

I’ve installed and edited the plugin succesfully and I’m very satisfied with it.
But I have one problem which I can’t solve by now.
After placing an resuest and clicking on “submit”, the website reloads and shows an “404” error page.

Can someone tell me why and how to solve this?

And as an extra there would be an wish:
Is it somehow possible to create an additional input field where people can also enter some greeting to their request?

Thanks in advance!