So, I can see what the problem is - when I do www.weider.es/fakepagetest it shows as a 404 in Fiddler. However when I do the same for tiendaweider it shows as a series of 302's (which are redirects so the page is redirecting to itself). if I look at one example I get the following
HTTP/1.1 302 Found
Cache-Control: no-cache
Pragma: no-cache
Content-Type: text/html; charset=utf-8
Expires: -1
Location: http://www.tiendaweider.es/fakepagetest
Server: Microsoft-IIS/8.0
X-Result-Reason: Not Redirected
X-UrlRewriter-404: 404 Rewritten to DNN Tab : Error 404(Tabid:689) : Reason Requested_404
Set-Cookie: dnn_IsMobile=False; path=/; HttpOnly
Set-Cookie: language=es-ES; path=/; HttpOnly
X-Powered-By: ASP.NET
Date: Sat, 18 Apr 2015 13:12:02 GMT
Content-Length: 156
<html><head><title>Object moved</title></head><body>
<h2>Object moved to <a href="http://www.tiendaweider.es/fakepagetest">here</a>.</h2>
</body></html>
As you can see by the x-urlrewriter-404 it is going to the handler as expected, and I can see it should redirect to tabid 689 - if I go directly to that (http://www.tiendaweider.es/default.as...) the page loads as expected. I can't see any reason why this isn't working I'm afraid, so you'll either have to debug it and work it out, or else not enable 404 for that portal.