Hi Markus,
The version of NCrunch you're using was coded with an incorrect value in the checking for the version of .NET installed. Although this worked fine at the time, when a later version of .NET was later installed the check became invalid and no longer worked correctly.
The correct solution here is to upgrade your version of NCrunch. It might be possible to work around the limitation by dropping back a version or uninstalling later versions of .NET, but I can't confirm this with certainty as I don't have reliable information around which versions are affected or are clashing. I published a fix for this issue as soon as it was reported.
Although we do our best to implement things in a forward compatible manner, combining old versions of NCrunch with newer versions of the .NET platform tooling (i.e. VS, .NET framework, .NET Core, MSBuild, test frameworks) will have a very high chance of exposing you to problems like this one. The platform has been moving very fast for many years, and new changes being made to it are not released as entirely separate compartments that are installed side-by-side (contrary to what the installers would have you believe). We can't warrant that older versions of NCrunch will continue to work if you update your environment. This means you'll either need to freeze your dev environment or keep everything updated.