An error occurred while validating. HRESULT = '80004005'

I was having trouble with this error today, when I was trying to build a Deployment package. I also noticed that my deployment package showed no Detected References anymore.
The weird thing was, that my solution build properly and I could even run the software. It was just the deployment package that was producing errors.

As I found, the first thing to check is missing references. If they are not in use in the code, debugging the project will not produce any errors, building the deployment will. This also includes references to other projects, which are not included in the solution. If not pointing directly to the pre-built dll, these will produce problems.

For me though, this did not solve my problem, but I did find something else that caught my eye;