Map Crash with error message "Method not found: '!!0[] System.Array.Empty()'"

I had my PC freshly installed and afterwards neither AutoCAD Map 2017 nor 3ds Max 2017 ran. Both crashed with error messages which looked like an .Net issue to me. Our IT department took my PC back and tried to reinstall - but to no avail. Apparently they had to change the harddisk to get both applications to run properly. 

Anyway - Map and Map-Administrator run now but only if I avoid loading a third party plugin. As soon as the plugin loads both programs crash - which doesn't happen on any other PC I tried. The CER report details show the following message:

<InnerException type="System.MissingMethodException"><Message>Methode nicht gefunden: "!!0[] System.Array.Empty()".</Message><StackTrace><Method>

It seems that the plugin was compiled against .net 4.6 but the .net runtime is lower than that. You can find a more detailed explanation here (reply by Alexandru).

That is a bit odd as Map 2017 itself requires .net 4.6 and therefore should not start up or run without 4.6 - but it does. When I checked the version of .net framework under "Installed Programs" it showed 4.6 but also 4.5:

The problem seems to be with language pack and / or registry settings.  Here are the two keys where the values don't match:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full\1031\Version = 4.6.00081
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full\1033\Version = 4.5.51209

On second PC both keys have the same value and under  "Installed Programs" it shows .net 4.6 twice:

Map 2017, SP1