WSUS Control Manager failed to monitor WSUS Server

SCCM console showing errors;

WSUS Control Manager failed to monitor WSUS Server "SCCB.W2016.LAB.

Possible cause: WSUS Server version 3.0 SP2 or above is not installed or cannot be contacted.
Solution: Verify that the WSUS Server version 3.0 SP2 or greater is installed. Verify that the IIS ports configured in the site are same as those configured on the WSUS IIS website.
Message ID: 7003

Followed by;
WSUS Control Manager failed to configure proxy settings on WSUS Server "SCCB.W2016.LAB".

Possible cause: WSUS Server version 3.0 SP2 or above is not installed or cannot be contacted.
Solution: Verify that the WSUS Server version 3.0 SP2 or greater is installed. Verify that the IIS ports configured in the site are same as those configured on the WSUS IIS website. You can receive failure because proxy is set but proxy name is not specified or proxy server port is invalid.

Messag ID: 7000


WSUSCtrl.log on the SUP;
System.Net.WebException: The request failed with HTTP status 503: Service Unavailable.~~   at Microsoft.UpdateServices.Administration.AdminProxy.CreateUpdateServer(Object[] args)~~   at Microsoft.SystemsManagementServer.WSUS.WSUSServer.ConnectToWSUSServer(String ServerName, Boolean UseSSL, Int32 PortNumber)
 Failures reported during periodic health check by the WSUS Server SCCB.W2016.LAB. 
Will retry check in 1 minutes SMS_WSUS_CONTROL_MANAGER

This check will happen every min so error will appear every min. aswell.

WSUSCtrl.log on the SUP is clearly indicating there is an issue with IIS service availability

The first point to check is the status of WSUS application pool.

Open IIS manager then go to Application pools then check WSUS pool.

In this instance my WSUS pool has stopped.
Once the service is started, the wsusctrl.log file check the connectivity and will successfully validate the connection.

To address the actual WSUSpool issue, consider changing the WSUS pool queue length in IIS from default 1000 to 25000

Follow blog from Microsoft to re-configure the Wsuspool queue length;
https://blogs.msdn.microsoft.com/the_secure_infrastructure_guy/2015/09/02/windows-server-2012-r2-wsus-issue-clients-cause-the-wsus-app-pool-to-become-unresponsive-with-http-503/