Error 503 is usually caused by 2 possible issues.
Cause 1 – App Pool issues
– The WsusPool Application Pool in IIS is in a Stopped state.
– The Private Memory Limit (KB) for the Application Pool is set to the default value of 1843200 KB.
To fix this:
Open the WAM Shell (Windows Menu > Programs > AJ Tek > WSUS Automated Maintenance > WAM Shell) and run:
.\Clean-WSUS.ps1 -SetApplicationPoolMemory 4096
Cause 2 – Bad or Incorrect Proxy Issues
– Check to see if there is a proxy setup.
From either the WAM Shell or an Administrator PowerShell instance run:
Write-Output “Proxy: $(netsh winhttp show proxy))”
Confirm that this is the correct setting for your network. If you require to enable the proxy, disable the proxy, or change the proxy, you can do so using Internet Options.
Once that’s done, open the WAM Shell and run: