Cannot Build Win 10 .NET (Il2CPP works, as does 8.1 .NET)

See title. For some reason, whenever I build a Win 10 Universal app using the Mono 2x or Win RT Dot NET scripting backends, I cannot compile the final product in Visual Studio. I receive thousands of errors implying that it does not even recognize basic data types, eg:

Code (csharp):

1.  
2. Severity    Code    Description    Project    File    Line    Suppression State
3. Error    CS0518    Predefined type 'System.String' is not defined or imported    Sound Box    C:\Users\devel_000\AppData\Local\Temp\.NETCore,Version=v5.0.AssemblyAttributes.cs    3    Active
4.  

Windows 10 Universal Il2CPP builds work great, however I need to work with some native Win 10 features and would really rather deal with a c# project than a C++ one.

Windows 8.1 Universal (Win RT dot Net scripting backend) works great, too, and previous C# builds worked fine in Unity 5.1.x


edit: Win 10 .NET builds work only in Debug mode, apparently. I will do some more digging to find out why this is