Ran into a situation today where after AD FS federation server was installed, configured and up & running, “all of a sudden” it stopped working.
Turned out that another installer that affected the default web site, also seemingly affected the AppPools associated to all Applications under the Default Web site.
By changing the “Enable 32-bit Applications” either through IIS admin or via command line
appcmd set apppool /apppool.name:MyAppPool /enable32BitAppOnWin64:false
Back to normal…