I encountered this issue in my lab, when trying to setup WSUS for my SCCM server (to use as a Software Update Point). So after installing the WSUS role (using a SQL database, and not the WID option), you will notice that there are some post-install configuration required.
When you click on the ‘Launch Post-Installation tasks’ link, the post-install tasks will be initiated.
But the post-config failed. When looking at the produced temp file, it showed the following.
2015-03-18 20:43:04 CreateDefaultSubscription failed. Exception: System.Net.WebException: The request failed with HTTP status 503: Service Unavailable.
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object parameters)
at Microsoft.UpdateServices.Internal.BaseApi.UpdateServer.CreateUpdateServer(String serverName, Boolean useSecureConnection, Int32 portNumber)
at Microsoft.UpdateServices.Internal.BaseApi.UpdateServer..ctor(Boolean bypassApiRemoting)
2015-03-18 20:43:04 StartServer encountered errors. Exception=The request failed with HTTP status 503: Service Unavailable.
2015-03-18 20:43:04 Microsoft.UpdateServices.Administration.CommandException: Failed to start and configure the WSUS service
at Microsoft.UpdateServices.Administration.PostInstall.Execute(String arguments)
Notice that it says Service Unavailable. I went to the Windows Server, and was able to start it. So what gives? Searching through the web, I came across many differing solutions, from command line configuration, to port modifications, etc. But none of these actually showed/had the same error that I did. There was mention of a known issue with WSUS, but that was on Windows Server 2012; my lab systems are all running on Windows Server 2012 R2.
I tried removing the WSUS role, and reinstalling, to no avail. Very frustrating, until I came across this post (http://myitforum.com/myitforumwp/2013/02/21/wsus-post-installation-configuration-task-failed/) which did show the exact error that I was encountering (HTTP status 503: Service Unavailable). This article mentioned that there might be an IIS issue. The solution suggested was to launch the IIS console, delete the existing WSUS Administration site, and re-initiate the post-installation tasks.
After I did this, the post-installation tasks completed successfully, and I was able to finish the setup of the SCCM Software Update Point with the WSUS sync working properly (as observed in the WCM.log file).
I hope this post helps someone, as it took me a while to find someone else that posted the exact same error/issue I encountered.