Lost access to Desktop Help of ArcMap 10.5?

I always use the Web rather than Desktop Help but I just tested the latter to see if I could reproduce what you describe in my configuration (which received 11 Windows updates on 10 Oct 2018).

On Windows 7 SP1, using ArcGIS Desktop 10.6, the Desktop Help starts fine from the Windows Start Menu, and I seem to be able to use that to navigate to any of its pages.

However, when I use the Tool Help button from any geoprocessing tool:

I get the same as you:

so I think you should report this to the Technical Support of your Esri International Distributor.