My solution contains a project and i added another existing project to the solution and did the "Add Reference" and "Manage imported projects" process. Then I made 'Set as StartUp Project' as first project. When i start debugging it, it shows an error "Invalid Project Path". Please see attached.
You'll need to provide more information. I suspect you are doing something else that you've not described. I can attach a sample project to project reference if you like, as it does work for me. Can you attach your solution? If not, I would suggest that you open a support request as it is likely we'd need to see it to assist.