I've tried all sorts of combos, but i can't get the 404 page to show. I only get the IIS standard 404.

 <customErrors mode="On" defaultRedirect="~/ErrorPage.aspx" />

does not work. If I visit this page directly I get 500 error, but that's maybe right?
I can't redirect to anything. I've tried to build a simple err.html and redirect to that, I've tried to redirect to a page created in dnn and that does not work, I've tried this as well:

<customErrors mode="On" defaultRedirect="~/ErrorPage.aspx" >
      <error statusCode="404" redirect="~/404.html" />
 </customErrors>

but this does not work either. Tried without "~" as well. Tried with [full url]/err.html whatever, it will just not redirect. What is wrong? First I was on7.02, now I'm on 7.04 with the same error, and now I'm ending up here.

The DNNFriendlyUrl is set to "Advanced", so I've tried without .aspx as well (for dnn created page). The redirect does just not work? Any ideas?

Well, so how can I make a custom 404 page?

<error statusCode="404" redirect="404error.aspx"/> should do the trick...? I'm quite new to DNN.