When using IIS as source repository for Alert Central's updates, you must consider that IIS by default blocks HTTP requests which contains escaped characters. Thus, when downloading the file libstdc++-4.4.7-3.el6.x86_64.rpm, IIS converts it to libstdc\\+\\+-4.4.7-3.el6.x86_64.rpm. This will trigger the following error message:
In order to fix this, we need to edit the web.config file located in the application root directory and add the bold text lines in configuration > system.webServer, as you can see in this simple example:
<?xml version="1.0" encoding="UTF-8"?>
<directoryBrowse enabled="true" />
<requestFiltering allowDoubleEscaping="true" />
Website needs to be restarted in order to read the new configuration. After the modifications, download works successfully.