Starting a new project in VisualStudio to test some cool library, pick a random name as project name, open manager NuGet dependencies and try to add the library.
Result: a stacktrace with Dictionary KeyNotFound. Try a few time and it happens every time. WTF.
Explanation: the random project name was the name of the library to test, resulting EXE is named after project and library DLL is named after DLL - and EXE and DLL with same name does not work well.
So try again with another poject name and everything works.
Now you also know!!