Sometimes it’s happen! when you submit a wordpress form it says ‘Page Not Found!’ or Saying ‘This is somewhat embarrassing, isn’t it?It seems we can’t find what you’re looking for. Perhaps searching, or one of the links below, can help.’
But your defined page action has a valid url and all post values are available too on that same page. Also if you submit that form as empty input that it works fine!
If this is the case then The problem is by the “name” parameter you posted from your form. WordPress has some reserve parameter for internal use like client_name, author_name etc
So you just need to try with renaming your form fields. Hope it will solve it!