Server: Microsoft HTTPAPI/2.0
A request comes into your server, it is picked up by http.sys (the kernel part of IIS) it doesn't know what to do with it and asks WAS (Windows activation service), WAS checks the configuration and figures out the web site and application pool to be used for this request. If the pool is not running, it is trying to start it.
If it can't start the pool, it reports back to http.sys: "sorry, I tried but I can't start the pool" At this point http.sys has no option but responding that the service for this request is not available.
If you look at the response header of your failed request, you will most likely see a
Maybe the request made it into the log file for the site, the http sub-status code next to the 503 may be helpful to know.
Most often, the pool can not be started due to authentication problems. Double check the identity of the pool. Use the security event log or Process Monitor to troubleshoot.
rather than a
that means the response comes from http.sys rather than the user mode part of IIS.