“Value does not fall within the expected range” when trying to add a reference in a project

Question

Suddenly I am getting this error \"Value does not fall within the expected range\" whenever I try to add a reference to any project. This also happens with a new blank proje

Answer 1

I have seen this box in VS-2019 16.5.4 in a Winforms/4.8 app with Std classlibs, on attempting to close one of the Winforms/4.8 source file tabs. The message came up, the tab refused to close.

Solved it by closing an re-opening the Solution.