I have seen many articles about this, but mainly for pre release versions of Visual Studio 2017. I have VS 2017 Professional version 15.3.3 installed.
As for myself, I can create new projects or open existing projects and all is fine, but if a colleague, passes me a copy of a project when I open it all of the references are shown with a yellow triangle. This includes anything from NuGet along with the NetStandardLibrary in the SDK dependency folder.
On opening up the reference manager and selecting Assemblies, it gets stuck on Assemblies - Targeting .Net Standard 1.5 (Colleagues existing project) with the message "Loading references", I am also not able to modify what assemblies I am able to look at.
The reference manager screen is not locked, as I can click on projects, Shard Projects, Com and Browse which all work, but on returning to the Assemblies tab, it is still just showing the message "Loading references".
@Martin-Ullrich : The point is there is no warning, just the yellow triangles.
However, you did touch upon the solution.
I installed VS 2017 Prof onto a Windows 10 Creator Edition PC which already had VS 2015 Prof installed.
Not sure if this caused the problem, but when checking from the Visual Studio installer, I found that the .Net Core Library had NOT been installed as standard.
On adding this to my local machine all started to work.
This is now the second machine that I have had this on. So be warned, that the Net Standard Libraries are not necessarily installed by default.