I'm doing some work with a customer that has + characters in their filenames. This was causing issues when trying to use Deep View Designer and specifying the filename to load.

The error I was getting when I put the full URL to the RH file into the DE address bar was:

HTTP Error 404.11 - Not Found
The request filtering module is configured to deny a request that contains a double escape sequence.

So, after doing some research around the web, I found the following site:
Learn IIS - top 10 changes in IIS7 

Item #8 defines the main issue with having + values in the URLs. So, I modified the following file:
%windir%\System32\inetsrv\config\applicationHost.config

<configuration>
  <system.webServer>
    <security>
      <requestFiltering allowDoubleEscaping="true"> ... </requestFiltering>
    </security>
  </system.webServer>
</configuration>

After restarting IIS, it worked great.