Out of the blue, starting getting:
Metadata file '...\abc\xyz.dll' could not be found
Look in the folder - there it is - WTF?
Mr. Google and figured out that something had changed in my build order.
To get the build order, right click the solution - "Project Build Order".
Then build all the dll first and then the projects that consume the .dll.
You can do this by playing around with the dependencies.