Our SSRS reporting server broke while troubleshooting another application. .NET was uninstalled and reinstalled which made SSRS very upset.
When browsing the ReportServer website I was getting a 403 Forbidden "This website requires you to login." The problem had nothing to do with what the error eludes to. The resolution was to re-align a couple things in IIS.
First, in the IIS manager go down to Web Service Extensions and make sure that ASP .NET is set to "Allowed."
Then, I had a missing entry in the virtual directory I needed to put back. Go to the ReportServer virtual directory under Web Sites. Right click -> properties -> configuration button. At the bottom, under "Wildcard application maps" make sure you have C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll set (if you're using asp .net 2.0). The path will be different for 1.1.
You may have to restart SSRS and IIS but that is what fixed it for me.