URL rewrite with IIS and FastCGI

We want to address a Python application (MapProxy) in an IIS 8.5 using FastCGI.On the Python page, wfastcgi.py is used.
This scenario works in principle, even with its own "MapProxy" site in IIS.

Our problem is that in this constellation we can no longer use URL rewrite to mask the calls to the MapProxy site.This is set on the "Default Web Site" and works with us only with "Redirect"."Rewrite" results in 404, although the URL pattern is correctly recognized and set.