On localhost, I've installed wordpress and the Betheme + all the plugins related to the theme and selected the Sample page as the 404 page and it shows the background only.
Yes, we use this plugin as well and everything works fine. There is probably something wrong with plugin settings on your side. This plugin is strongly related with server and settings we use on our demo, may not work for you and vice versa. So you must look for a problem inside plugin's settings.
It cannot be W3 Total Cache because I've tested on a clean install - Just the Betheme+ the plugins bundled and when I setup the 404 page to the Sample Page - it displays just the background image.
Ok, but how can we check error page? Any page we want to open, gives "Internal Server Error". Maybe will be better if you can send us private message with access to the dashboard and we'll check it from the back-end.
Comments
Please open your website in Firefox html mode (CTRL+U) and check html errors. Please fix them first and all should be fine.
BTW We see the page with feedback on the right side and wooden background. So it's not a blank page.