All project references have yellow triangles every time solution is loaded

Question

All my project (from the same solution) references are marked with a yellow triangle. The projects are all set to the same .NET version (4.5.1). The error log says:

Answer 1

I had the same problem as you can see in this picture.

Its reason was the hidden folder. I had hidden the folder containing the project files. I got rid of this problem by unchecking the hidden attribute of the root folder containing the project files and restarting the visual studio. Check it once, you might have checked the folder to be hidden.