error MSB3644: The reference assemblies for framework ".NETFramework,Version=v4.5" were not found

Meanwhile I've solved the issue, and it comes down to a general truth in working with computers:

When you have a problem, don't listen to what your computer tells you, but ask yourself what you were doing when the problem arose.

In my case, it means the following:
The problem popped up when I had upgraded my Visual Studio version (using the Visual Studio Installer), so the solution consisted of starting that Visual Studio Installer again, and check some more features to be installed (I've now checked so many of them that I can't figure out which one exactly solved my problem).

The error message, preceeded by the dialog box, were just information about the PC internals which only led to ununderstandable sidetracks.