Don't manage to build for windows UWP

hi

I am using Unity 5.5 and I am trying to build for UWP with .NET scripting backend a project that runs fine in the editor. When I do a change in code though (like adding a line), and return to Unity, the console throws a whole bunch of errors, the first one being

 Loading script assembly "Library/ScriptAssemblies/Assembly-UnityScript-Editor-firstpass.dll" failed!

and then

 FileNotFoundException: Could not load file or assembly 'System.Runtime, Version=4.0.20.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.

and then dozens of

 TypeLoadException: Could not load type 'LTDescr' from assembly 'Assembly-CSharp-firstpass, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.

Why/when do these errors occur since I can run in the editor? How can I find more info about them?

When I press "clear", the errors don't stay in the console and the project runs fine in the editor, so they're not code errors.

Then when I try to build, I get errors about the Hashtable type:

 Reference to type 'Hashtable' claims it is defined in 'mscorlib', but it could not be found.

So I have added WinRTLegacy.dll and set it for UWP SDK and .net. If I write

 #if NETFX_CORE
 using Hashtable = WinRTLegacy.System.Collections.Hashtable;
 #endif

I am told there is no "System" namespace in "WinRTLegacy". If I write

 #if NETFX_CORE
 using Hashtable = WinRTLegacy.Collections.Hashtable;
 #endif

I am told there is no "Collections" namespace in "WinRTLegacy". And if I don't use any #if instruction, I am told the Hashtable exists in both places.

Any help appreciated. Many thanks.