Warning with Unity 2018.1.3 and Visual Studio 2017

Hi, I have a question about Unity 2018.1.3 and Visual Studio 2017.

Every time I open Visual Studio with Unity 2018.1.3 by clicking on “Assets → Open C# Project”, I get the following warning: 119605-warning-project.png

This is very annoying as it happens every time I open VS from Open C# Project or by opening a script from Unity…

The solution view shows me two projects after I closed the warning, weird thing is, that I do not have any Editor scripts in my project at the moment.

I have already tried deleting all *.sln and *.csproj files and the .vs folder which did not change anything.
Does someone have an idea how to resolve this?

The problem seems to have gone after updating Visual Studio and / or installing an extension to Visual Studio.
Anyhow, now the solution behaves normally.