I am sorry to spam this forum but I am all out of ideas once again.
What I did:
New game,
Update package -> Add Code library
Named library OrangeSystem.Core
Opened Solution in VS 2015
Added .cs files files to the OrangeSystem.Core project
Installed NCalc from "Nuget opackage manager"
In project OrangeSystems.Game References added reference to project OrangeSystem.Core
Saved everithing build now fails with:
scereenshot
Those messages give me absolutely no clue what is wrong.