The error is coming from IIS. There are several additional layers which could go down inc. CGI, Application Pool(s), Database(s), parts of the LAN (interconnect

The error is coming from IIS. There are several additional layers which could go down inc. CGI, Application Pool(s), Database(s), parts of the LAN (interconnecting different servers/services), or anything else the ASP.NET site relies upon (e.g. handle depletion, disk space, etc). In IIS this is a "last resort" error when even the custom error handler cannot respond.