IIRF uses a URL of the form proxy.iirf?something to transfer some URL requests from the IIS filter to the IIS extension. This is done for PUT and POST requests that include a payload. A login form might be one example. Normally the transfer of
some requests form the IIS filter to the IIS extension is not something you need to be concerned about, but it appears to be breaking in this case.
You can see this problem if you have partially installed IIRF. For example if you've installed IIRF as an IIS Filter, but have not installed IIRF as an IIS Extension, this problem can occur. If you have used the IIRF setup program, then it will have
installed itself properly. If you have installed IIRF manually, then you would have needed to set it up both as an IIS Filter and as an IIS Extension. If you installed IIRF manually and don't understand what I mean, then you probably have not properly
set up IIRF.
Another possibility is that you have rules in the IIRF.ini file that prevent the proxy.iirf URLs from being properly served by IIRF.
The best way to see what is happening is to examine in the IIRF log. You should have logging set to 3 to do this.