Why do I get “An error occurred while validating. HRESULT = '80004005'” when building a setup project?

Question

Why do I get the following error when building a setup project?

An error occurred while validating. HRESULT = \'80004005\'

Answer 1

In my case my solution (VS2008) had a project that was also referenced from another solution (VS2010). In the VS2010 solution I had upgraded the project to .NET 4.0. When I later realized the project was also used in another solution I downgraded it to .NET 3.5. For some reason everything seemed to be changed correctly in the csproj file except one place which is mentioned here: Error in setup project HRESULT = '80004005'

I know this is already solved somewhere else but I wanted to shed some light on this from another problem angle.