Failed to load toolbox item. It will be removed from the toolbox

I was having same issue but I find the solution:
Click on "Project_Name" with left mouse and click "Build" then you can add the UserControl to your WinForm.

Change to AnyCPU and rebuild the project. Visual Studio has problems with controls being 64bit.

I had the same issue lately. As this (nor any other answer on this site and on the internet) actually helped me, I found out the way to fix it.
Just clean the files and rebuild the solution. As simple as that.

I was suffering from the same issue for one of our applications and could not find a solution. So I created the user and custom controls manually. After searching the web again I finally found why the designer is failing for this particular project. The answer is that project is 64 bit and Visual Studio is still does not have a 64 bit version, it is still 32 bit. So the designer could not load the controls in 64 bit applications or in class libraries. Read Microsoft's article about this. There was an article on Visual Studio web site but it was removed. See the the answer in Visual Studio support forum.

https://social.msdn.microsoft.com/Forums/vstudio/en-US/77e10b58-43cc-4aab-919f-888f14f99571/x64-class-library-of-user-controls?forum=csharpgeneral