We have a single internal AD FS 2.0 server and a single AD FS 2.0 proxy server. I am not sure when it happened, but the AD FS 2.0 proxy server sign-on page is blank. The page I'm referring to is https://<fqdn>/adfs/ls/ldpInitiatedSignOn.aspx.
I tried revoking the proxy and then going through the wizard on the proxy again, but this did not help. I also deleted the IIS app with the following command and then deleted the files in the folder on the file system and went through the wizard on the proxy again with the same result:
C:\Windows\System32\inetsrv\appcmd delete app /app.name:"Default Web Site"/"adfs/ls"
The site used to work fine. We are not sure when exactly it broke as the AD FS proxy is used externally very sparingly by our employees.