Is it normal that Scripts don't compile in Visual Studio while they do in Unity itself

Questions & Answers
1

On my mac i went from the 2017.3 stable release to the beta version 2018.1.0b13 and therefore also from Mono Develop to Visual Studio.

While being able to compile and check scripts in Mono Develop and Unity, i can’t do so in Visual Studio. When attempting to even compile the simplest of scripts i get error messages like:

    CSC : error CS2001: Source file '/Users/simon/Desktop/unity/simonfly/Packages/com.unity.package-manager-ui/Editor/Sources/Services/Common/ThreadedDelay.cs' could not be found.

38 of these to be exact. I just don’t get why it is searching for all these files in my project folder.
All of this applies even for a vanilla project with one default script.

2

It is not normal!
Somehow my first installation got screwed up and while reinstalling Visual Studio i never deleted stuff from the Library folder.
Following this Deinstallation Guide and reinstalling Visual Studio solved my issues.