[Plugin: 404page] Error page doesn't work for subpages

Dear WordPress people,

with 404page I found a nice plugin which allowed me to create a nice custom error page four our website.

Does anyone know how I get this page to work for the subpages http://test.gcconsulting.de/expertise and http://test.gcconsulting.de/team too?

I followed the steps demanded by the login:
1) create custom page with title 404 error page
2) select this page via settings

Yet on the subpage only a blank page or the regular wordpress “page not found” page appear.

Happy to read from you : -)