The WSUS engine had crashed and it turns out the problem is it runs out of memory. Make sure your WSUS server had at least 8GB of memory then perform the following:

1. Open IIS Manager
2. Select <server> – Application Pools
3. Right click on WsusPool and select Advanced Settings
4. Change the Recycling – Private Memory Limit (KB) from 1.4GB to around 4.8GB, e.g. 50331645
5. Click OK
6. Start or Recycle the WsusPool

Also I changed the values of the WSUS Application Pool in IIS (Advanced Settings).

• Queue Length: 25000 from 1000 Limit
• Interval (minutes): 15 from 5 “Service Unavailable”
• Response: TcpLevel from HttpLevel